Well, OK, I guess it's more over than up...
I have migrated to a new blog address. All the old posts from here were moved over, so they're still available on the new blog. I will not be adding any new posts to this site, all the new ones should be on:
http://chemistryingeneral.blogspot.com/
See you on the new blog, I may still fiddle around with the colors a bit...
2013/02/26
2013/02/23
Keeping track of concentrations
When doing kinetics and equilibrium problems, especially when you're doing an actual hands-on experiment, there are a lot of different concentrations to keep track of. The key to keeping them straight is mostly careful reading and organization, but there are a couple common definitions or descriptions that can help.
Stock Concentration
This is the one that comes up most common in a lab experiment. "Stock" refers to the large samples of reagent from which smaller amounts are taken for individual experiments. When you come to lab, the big bottles or carboys of solution that are on the side benches or in the dispensing hood are "stock" solutions and should have a "stock concentration" listed on the bottle. Most data analysis in lab begins with stock concentrations.
Initial Concentration
In either kinetics or equilibrium problems or experiments, we will often come upon something called an initial concentration. "Initial concentration" is (to me at least) quite fascinating because it's one of those things that we can do on paper that's just not physically possible to do in the real world. The "initial concentration" in a problem or experiment is the concentration of reactants after mixing everything together but before any reaction is allowed to take place. It's as if there was a little "start reaction" button on the side of the beaker, and nothing reacted until we pushed that button. In the real world, as soon as reactant solutions come in contact with each other, they begin to react, so the "initial concentration" is never the actual concentration we might observe in a reaction mixture. The initial concentration is most commonly calculated as a dilution of the stock concentration.
{In some specific reactions, we can probably observe an initial concentration because either the reaction is SO slow that we can mix the reactants before any measurable reaction has occurred, or because there's some external stimulus (like light or heat) required to make the reaction start. These reactions aren't that rare, but they're not reactions that we're likely to use very often in Gen Chem.}
Partial Pressures
When we're working with gases, we can often use Molarity to express the concentration of reactants, but we can also use partial pressures of the gaseous components. Partial pressures are a way to measure the number of moles of a specific gas in a mixture of gases. {Dalton's Law of Partial Pressures} That sounds an awful lot like a concentration... As with the vast majority of data analysis in chemistry, counting moles is the key to figuring out relationships between reacting species. If we know the concentration and volume of a liquid solution, we'll probably be calculating moles at some point. If we know the partial pressure of a gas, we'll probably also be calculating moles at some point. Chemistry is all about the mole!
The most important thing to do when approaching these problems is organization. This is especially true of equilibrium problems; if we can organize the information given in the problem, we'll be much more successful.
Did you hear about the chemist who was thinking very hard about removing excess solvent from a solution? She was concentrating.
Stock Concentration
This is the one that comes up most common in a lab experiment. "Stock" refers to the large samples of reagent from which smaller amounts are taken for individual experiments. When you come to lab, the big bottles or carboys of solution that are on the side benches or in the dispensing hood are "stock" solutions and should have a "stock concentration" listed on the bottle. Most data analysis in lab begins with stock concentrations.
Initial Concentration
In either kinetics or equilibrium problems or experiments, we will often come upon something called an initial concentration. "Initial concentration" is (to me at least) quite fascinating because it's one of those things that we can do on paper that's just not physically possible to do in the real world. The "initial concentration" in a problem or experiment is the concentration of reactants after mixing everything together but before any reaction is allowed to take place. It's as if there was a little "start reaction" button on the side of the beaker, and nothing reacted until we pushed that button. In the real world, as soon as reactant solutions come in contact with each other, they begin to react, so the "initial concentration" is never the actual concentration we might observe in a reaction mixture. The initial concentration is most commonly calculated as a dilution of the stock concentration.
{In some specific reactions, we can probably observe an initial concentration because either the reaction is SO slow that we can mix the reactants before any measurable reaction has occurred, or because there's some external stimulus (like light or heat) required to make the reaction start. These reactions aren't that rare, but they're not reactions that we're likely to use very often in Gen Chem.}
Partial Pressures
When we're working with gases, we can often use Molarity to express the concentration of reactants, but we can also use partial pressures of the gaseous components. Partial pressures are a way to measure the number of moles of a specific gas in a mixture of gases. {Dalton's Law of Partial Pressures} That sounds an awful lot like a concentration... As with the vast majority of data analysis in chemistry, counting moles is the key to figuring out relationships between reacting species. If we know the concentration and volume of a liquid solution, we'll probably be calculating moles at some point. If we know the partial pressure of a gas, we'll probably also be calculating moles at some point. Chemistry is all about the mole!
The most important thing to do when approaching these problems is organization. This is especially true of equilibrium problems; if we can organize the information given in the problem, we'll be much more successful.
Did you hear about the chemist who was thinking very hard about removing excess solvent from a solution? She was concentrating.
Suggested Problems posted
A few people have asked about some suggested end-of-chapter problems from your book. I've picked some from our current chapters (http://www.drbodwin.com/teaching/genchem.php), this should be a good list for you to start with. Use these to identify the areas that you need to study/learn/review a little more, work through additional problems as needed. And always ask questions if you're unsure.
2013/02/16
Gen Chem Pre-Lab poll
For the past few years, I have used video pre-labs (posted on YouTube) as a way to introduce students to the week's experiment in General Chemistry Labs. There were number of reasons for this, and now that I've done it for a while, I'd like to get some feedback. There's a poll on this blog (look left, right above the fish...) to collect your opinions/preferences. If you'd like to leave additional comments, leave them here, that's why I put up a post as well as the poll. I'm not trolling for praise, I really just want to know how useful the video pre-labs are compared to some other options. Setting up, shooting, editing and posting the videos takes a little bit of time and effort, and I kind of enjoy doing it, but if students don't consider it a positive contribution to the lab experience, I could probably re-direct my effort toward other projects.
2013/02/09
Exam manners
I'd like to start by apologizing for my behavior at the end of the exam. I admit that I got a little grumpier than I should have, and I will do my best to never let it happen again. Part of the problem is that I did not make my expectations clear about what I expect when students take exams in my class, so let me try to clarify.
As your instructor, I feel it is my responsibility to provide you the best possible environment in which to take your exams. We take our exams in a large room so that everyone has space. I try to be as quiet as possible while I'm walking around the room. I write time reminders up on the board for those of you who may not be able to see the clock from your seat. I try to provide an environment that is as free from distractions as possible without putting everyone in their own individual, noise-proof rooms.
In return, I expect a few things from you.
1. Arrive on time. It is disruptive to your classmates to have people trickling in, opening and closing doors, plopping down in their seats, and otherwise making noise that is distracting. I understand that there are legitimate reasons for being late, but I doubt that everyone who wandered in late had a good reason. If you KNOW you might be late for a legitimate reason, let me know in advance.
2. Turn OFF your phone. You're not going to answer it during the exam, are you? Many phones vibrate rather loudly when you're in a (hopefully) quiet room, so just turning off the ringer can still be very disruptive to the students sitting near you, trying to concentrate and perform well on the exam.
3. When you finish the exam, either leave or be silent. If there are still people taking the exam and you decide to have a chat with your friend in the exam room, you are being disruptive. In fact, you are being an inconsiderate jerk. In the future, if I hear or see anyone talking in the exam room, I will assume you are trying to cheat in some way and you will fail the course. Even if you've already handed in your exam, I will assume you are trying to cheat and you will fail the course. I understand that many of you have class in SL104 right after our class, so it seems natural to just stay in the room, but if you want to talk to anyone other than me, do it in the hallway.
All of these things can be pretty easily summed up as "Don't do anything that is intentionally disruptive to the other students in the room". That doesn't seem like an unreasonable request to me. I would hope that all of your classmates would behave similarly. If everyone is just a LITTLE considerate of the rest of the people in the room, then EVERYONE can have a more positive and productive exam experience. A couple other tips fall under the "Be prepared and responsible for your own success" category:
1. If you're taking an exam, there's a pretty good chance that you'll need something to write with. I do not use Scantrons for the regular exams in my Gen Chem classes, so it doesn't matter whether you use pencil or pen for regular Gen Chem exams. Pencils and pens are not all that expensive, so it's not a bad idea to have one of each with you. In fact, it's not a bad idea to have 3 or 4 of each with you.
2. If you're feeling a little sniffly or prone to coughing, bring a small pack of tissues and/or some cough drops with to class. {NOTE: If you're using tissues in class, please dispose of them properly. Every semester there's at least one person who throws used tissues on the floor or under their seat in SL104 and the janitors have to clean them up. That's gross. Don't do it.} If your cough or throat tickle is persistent, you might even bring a bottle of water to sip during the exam or class. Speaking of which...
3. Think before you drink. This applies to evening parties, but it's also good advice for early mornings. For most people, if you drink a quart of coffee or soda or juice or water or Red Bull, there's a pretty good chance that all of that liquid will require a stop in a restroom. It's best if this does not occur in the middle of an exam, especially when the exam only lasts 50 minutes. A quick stop in the restroom before the exam starts will let you think about the exam without wondering if your teeth really are literally floating out of your mouth.
4. Bring an appropriate calculator. My calculator policy is in the syllabus and is there thanks to one of your predecessors who downloaded most of the textbook into his TI-84 graphing calculator. In the middle of an exam, I stood looking over his shoulder as he scrolled through the text. I might have a "loaner" calculator with me at the exam, but don't expect it. Similar to the pen/pencil comment above, it wouldn't be the wackiest idea in the world to think about bringing 2 calculators to class, especially since the calculators you are allowed to use on my exams can cost less that $10. I noticed a couple people using extremely basic check-book style calculators during the exam that can't do anything more than add, subtract, multiply and divide. These probably cost $1-2 and would be a GREAT "backup" calculator to have in your bag.
5. Don't ignore the clock. But don't obsess about the clock for the whole 50 minutes. I typically don't audibly announce how much time is left until the 10 minute mark because it's disruptive to students. When I announce "10 minutes left", it means you have some time left to work through a problem or two. When I announce "2 minutes left", that means you might have time to finish the problem you're working on and give a quick look through the exam to make sure your name is on every page and you've answered any multiple choice questions that might be on the exam (even if you have to guess). If I have to announce "30 seconds left", that means you need to get out of your seat and bring me your exam NOW. There is another class waiting to come in the room, and I need to clear out to allow the next class to start on time. I will not come to you and take the exam out of your hands, I will just leave the room and you will earn a score of zero on the exam.
For those of you who have read this far, congratulations, you must actually care about your success in my class and in college and in life. In many cases, you are probably also not the students who really need to read this because you are already considerate of your classmates and doing your best to be prepared and responsible. The vast majority of you are good students, and I thank you for everything you do. You make class better by being there. Have a good weekend and I'll see you on Monday.
As your instructor, I feel it is my responsibility to provide you the best possible environment in which to take your exams. We take our exams in a large room so that everyone has space. I try to be as quiet as possible while I'm walking around the room. I write time reminders up on the board for those of you who may not be able to see the clock from your seat. I try to provide an environment that is as free from distractions as possible without putting everyone in their own individual, noise-proof rooms.
In return, I expect a few things from you.
1. Arrive on time. It is disruptive to your classmates to have people trickling in, opening and closing doors, plopping down in their seats, and otherwise making noise that is distracting. I understand that there are legitimate reasons for being late, but I doubt that everyone who wandered in late had a good reason. If you KNOW you might be late for a legitimate reason, let me know in advance.
2. Turn OFF your phone. You're not going to answer it during the exam, are you? Many phones vibrate rather loudly when you're in a (hopefully) quiet room, so just turning off the ringer can still be very disruptive to the students sitting near you, trying to concentrate and perform well on the exam.
3. When you finish the exam, either leave or be silent. If there are still people taking the exam and you decide to have a chat with your friend in the exam room, you are being disruptive. In fact, you are being an inconsiderate jerk. In the future, if I hear or see anyone talking in the exam room, I will assume you are trying to cheat in some way and you will fail the course. Even if you've already handed in your exam, I will assume you are trying to cheat and you will fail the course. I understand that many of you have class in SL104 right after our class, so it seems natural to just stay in the room, but if you want to talk to anyone other than me, do it in the hallway.
All of these things can be pretty easily summed up as "Don't do anything that is intentionally disruptive to the other students in the room". That doesn't seem like an unreasonable request to me. I would hope that all of your classmates would behave similarly. If everyone is just a LITTLE considerate of the rest of the people in the room, then EVERYONE can have a more positive and productive exam experience. A couple other tips fall under the "Be prepared and responsible for your own success" category:
1. If you're taking an exam, there's a pretty good chance that you'll need something to write with. I do not use Scantrons for the regular exams in my Gen Chem classes, so it doesn't matter whether you use pencil or pen for regular Gen Chem exams. Pencils and pens are not all that expensive, so it's not a bad idea to have one of each with you. In fact, it's not a bad idea to have 3 or 4 of each with you.
2. If you're feeling a little sniffly or prone to coughing, bring a small pack of tissues and/or some cough drops with to class. {NOTE: If you're using tissues in class, please dispose of them properly. Every semester there's at least one person who throws used tissues on the floor or under their seat in SL104 and the janitors have to clean them up. That's gross. Don't do it.} If your cough or throat tickle is persistent, you might even bring a bottle of water to sip during the exam or class. Speaking of which...
3. Think before you drink. This applies to evening parties, but it's also good advice for early mornings. For most people, if you drink a quart of coffee or soda or juice or water or Red Bull, there's a pretty good chance that all of that liquid will require a stop in a restroom. It's best if this does not occur in the middle of an exam, especially when the exam only lasts 50 minutes. A quick stop in the restroom before the exam starts will let you think about the exam without wondering if your teeth really are literally floating out of your mouth.
4. Bring an appropriate calculator. My calculator policy is in the syllabus and is there thanks to one of your predecessors who downloaded most of the textbook into his TI-84 graphing calculator. In the middle of an exam, I stood looking over his shoulder as he scrolled through the text. I might have a "loaner" calculator with me at the exam, but don't expect it. Similar to the pen/pencil comment above, it wouldn't be the wackiest idea in the world to think about bringing 2 calculators to class, especially since the calculators you are allowed to use on my exams can cost less that $10. I noticed a couple people using extremely basic check-book style calculators during the exam that can't do anything more than add, subtract, multiply and divide. These probably cost $1-2 and would be a GREAT "backup" calculator to have in your bag.
5. Don't ignore the clock. But don't obsess about the clock for the whole 50 minutes. I typically don't audibly announce how much time is left until the 10 minute mark because it's disruptive to students. When I announce "10 minutes left", it means you have some time left to work through a problem or two. When I announce "2 minutes left", that means you might have time to finish the problem you're working on and give a quick look through the exam to make sure your name is on every page and you've answered any multiple choice questions that might be on the exam (even if you have to guess). If I have to announce "30 seconds left", that means you need to get out of your seat and bring me your exam NOW. There is another class waiting to come in the room, and I need to clear out to allow the next class to start on time. I will not come to you and take the exam out of your hands, I will just leave the room and you will earn a score of zero on the exam.
For those of you who have read this far, congratulations, you must actually care about your success in my class and in college and in life. In many cases, you are probably also not the students who really need to read this because you are already considerate of your classmates and doing your best to be prepared and responsible. The vast majority of you are good students, and I thank you for everything you do. You make class better by being there. Have a good weekend and I'll see you on Monday.
2013/02/07
Improving your lab scores
A few trends are emerging that are causing people to lose points in lab.Hopefully this will help.
Pre-Lab Quizzes:
1. Do them. One of the fastest ways to fail Gen Chem lab is to skip quizzes.
2. Look at them early enough to ask questions. Pre-lab quizzes are due by 8:00am on Thursdays this semester. If you email me a question at 11:45pm on Wednesday night, I will not see it or answer it in time to be helpful for you. Please do not interpret this as "Don't ask questions." I WANT you to ask questions if you don't understand a problem, but it's more effective to ask those questions on Monday or Tuesday.
3. Read the questions carefully. This actually applies to any situation in any class. There are ALWAYS little clues in the way a question is asked. They may be subtle at times, but they're there. For example, "assume volumes are additive" probably means that you might have to add some volumes together at some point. There may be times that I include information that is NOT necessary to solve a problem, but I definitely don't go out of my way to put a bunch of non-essential information in a problem, especially when it's an online quiz question.
Hand-In Assignments:
1. Circle, highlight, or otherwise clearly indicate who your Lab Assistant is on everything. When I'm trying to sort 100+ assignments, it's much easier if you've obviously done this. On future assignments, if you do not clearly and obviously indicate your Lab Assistant, you will lose points.
2. All assignments must be typed. This includes sample calculations. There are only 2 (maybe) exceptions to this: experimental set-up diagrams in lab reports, and questions that ask you to label things on a printed graph. In both of these cases, it would be BETTER to do everything electronically so you have a back-up copy to print. ("I lost my only copy of the graph as I was walking to Hagen to hand in my assignment 5 minutes before it was due.") If you don't know how to use an equation editor in MSWord or other word processing programs, look into it. They're great tools and can make really nice looking sample calculations.
3. Read the questions carefully and answer them completely. If your answer to a question takes fewer words than the actual question, you're probably not answering the question completely. Especially in lab, there are very few questions that have "Yes" or "No" as an answer. Always explain at least a little bit why the answer is "Yes" or "No".
Look back through old posts on this blog; this isn't the first time I've posted tips on how to improve scores. Use the resources you have available to you.
Pre-Lab Quizzes:
1. Do them. One of the fastest ways to fail Gen Chem lab is to skip quizzes.
2. Look at them early enough to ask questions. Pre-lab quizzes are due by 8:00am on Thursdays this semester. If you email me a question at 11:45pm on Wednesday night, I will not see it or answer it in time to be helpful for you. Please do not interpret this as "Don't ask questions." I WANT you to ask questions if you don't understand a problem, but it's more effective to ask those questions on Monday or Tuesday.
3. Read the questions carefully. This actually applies to any situation in any class. There are ALWAYS little clues in the way a question is asked. They may be subtle at times, but they're there. For example, "assume volumes are additive" probably means that you might have to add some volumes together at some point. There may be times that I include information that is NOT necessary to solve a problem, but I definitely don't go out of my way to put a bunch of non-essential information in a problem, especially when it's an online quiz question.
Hand-In Assignments:
1. Circle, highlight, or otherwise clearly indicate who your Lab Assistant is on everything. When I'm trying to sort 100+ assignments, it's much easier if you've obviously done this. On future assignments, if you do not clearly and obviously indicate your Lab Assistant, you will lose points.
2. All assignments must be typed. This includes sample calculations. There are only 2 (maybe) exceptions to this: experimental set-up diagrams in lab reports, and questions that ask you to label things on a printed graph. In both of these cases, it would be BETTER to do everything electronically so you have a back-up copy to print. ("I lost my only copy of the graph as I was walking to Hagen to hand in my assignment 5 minutes before it was due.") If you don't know how to use an equation editor in MSWord or other word processing programs, look into it. They're great tools and can make really nice looking sample calculations.
3. Read the questions carefully and answer them completely. If your answer to a question takes fewer words than the actual question, you're probably not answering the question completely. Especially in lab, there are very few questions that have "Yes" or "No" as an answer. Always explain at least a little bit why the answer is "Yes" or "No".
Look back through old posts on this blog; this isn't the first time I've posted tips on how to improve scores. Use the resources you have available to you.
2013/01/23
Stoichiometry (Calcium iodate pre-lab quiz)
Lots of people with questions about the Calcium Iodate Pre-Lab Quiz. These are stoichiometry problems and require practice, practice, practice. If you didn't get enough practice in Gen Chem I, you should practice, practice, practice some more, then practice again.
How to do a stoichiometry/limiting reagent problem:
1. Write a balanced chemical equation.
2. Convert a known amount to moles.
3. Using the relationships in the balanced equation, convert the moles of the known substance to moles of the substance you're looking for.
4. Convert the moles of the substance you're looking for into whatever quantity you'd like to know.
Those are the same 4 steps you'll use for every stoichiometry problem you will ever do. The details will change, but the basic steps are the same.
12.5grams of nitrogen gas reacts with 11.6g of hydrogen gas to form ammonia gas. What is the theoretical yield of ammonia gas in grams?
Step 1: N2(g) + 3 H2(g) --> 2 NH3(g)
If nitrogen is the limiting reagent...
Step 2: (12.5g N2) / (28.014g/mol) = 0.4462mols N2(g)
Step 3: (0.4462mols N2(g)) (2 mols NH3(g) / 1 mol N2(g)) = 0.8924mols NH3(g)
Step 4: (0.8924mols NH3(g)) (17.031g/mol) = 15.2g NH3(g)
But we don't really know if nitrogen is the limiting reagent, so...
If hydrogen is the limiting reagent...
Step 2: (11.6g H2) / (2.0158g/mol) = 5.7545mols H2(g)
Step 3: (5.7545mols H2(g)) (2 mols NH3(g) / 3 mol H2(g)) = 3.8364mols NH3(g)
Step 4: (3.8364mols NH3(g)) (17.031g/mol) = 65.3g NH3(g)
What can you do besides practice, practice, practice stoichiometry problems to get better at them? Always, always, always write out your units and make sure they cancel out correctly. If your units to not cancel out correctly, it is exceedingly unlikely that you are setting up the problem correctly. Units are a wonderful little gift that lets you check your own work, I use them for every problem I set up.
Good luck and practice, practice, practice, practice, practice, practice, practice, practice, practice.
How to do a stoichiometry/limiting reagent problem:
1. Write a balanced chemical equation.
2. Convert a known amount to moles.
3. Using the relationships in the balanced equation, convert the moles of the known substance to moles of the substance you're looking for.
4. Convert the moles of the substance you're looking for into whatever quantity you'd like to know.
Those are the same 4 steps you'll use for every stoichiometry problem you will ever do. The details will change, but the basic steps are the same.
12.5grams of nitrogen gas reacts with 11.6g of hydrogen gas to form ammonia gas. What is the theoretical yield of ammonia gas in grams?
Step 1: N2(g) + 3 H2(g) --> 2 NH3(g)
If nitrogen is the limiting reagent...
Step 2: (12.5g N2) / (28.014g/mol) = 0.4462mols N2(g)
Step 3: (0.4462mols N2(g)) (2 mols NH3(g) / 1 mol N2(g)) = 0.8924mols NH3(g)
Step 4: (0.8924mols NH3(g)) (17.031g/mol) = 15.2g NH3(g)
But we don't really know if nitrogen is the limiting reagent, so...
If hydrogen is the limiting reagent...
Step 2: (11.6g H2) / (2.0158g/mol) = 5.7545mols H2(g)
Step 3: (5.7545mols H2(g)) (2 mols NH3(g) / 3 mol H2(g)) = 3.8364mols NH3(g)
Step 4: (3.8364mols NH3(g)) (17.031g/mol) = 65.3g NH3(g)
Since the nitrogen gas is all used up by the time we've made 15.2g of ammonia, there's no way we could make 65.3g of ammonia, so nitrogen must be the limiting reagent and the theoretical yield is 15.2g of ammonia gas.
What can you do besides practice, practice, practice stoichiometry problems to get better at them? Always, always, always write out your units and make sure they cancel out correctly. If your units to not cancel out correctly, it is exceedingly unlikely that you are setting up the problem correctly. Units are a wonderful little gift that lets you check your own work, I use them for every problem I set up.
Good luck and practice, practice, practice, practice, practice, practice, practice, practice, practice.
2012/12/04
Analysis of Iron Ore - question
Question from email:
If you break this calculation down into its parts, it's not quite as intimidating as it might seem.
Step 1 - Using your Beer's Law plots, calculate the iron concentration in the orange solutions you made in the second week of lab.
Step 2 - How did you make those orange solutions? You should have made them from 1.00mL of the yellow iron solution you made by dissolving your iron ore sample (red powder) in acid. You can use C1V1=C2V2 to determine the concentration of iron in the yellow solution.
Step 3 - Once you know the concentration (moles of iron per liter of solution) of the yellow iron solution, you can calculate how many moles of iron are present in that solution. You made 100.0mL of the yellow solution from your red iron ore sample.
Step 4 - If you know how many moles of iron were present in the yellow iron solution, you know how many moles of iron were present in the red iron ore powder. {Hint: they just might be the same...}
Step 5 - You know how many moles of iron were present in the red iron ore sample, using the Periodic Table you can calculate how many grams of iron were in the red iron ore sample.
Step 6 - If you know how many grams of iron were present in the red iron ore sample, and you know the total mass of the red iron or sample, then you should be able to calculate the percent (by mass) of iron that was present in the red iron ore sample.
For the iron ore samples you all analyzed, the percent iron could have been as low as 9-11% or as high as 33-35%. If you get something lower than 9% or higher than 35%, double check your calculations.
I'm working on the hand in right now and I'm just a little stuck on the % Fe question. I have all of the molarities calculated out. Do I simply take the molarity of the samples and divide them by .100 L for the moles and then go to grams to percent? My percentages just seem so small. I'm getting numbers such as .22% Fe. Or do I use the .050 L instead of .100.
If you break this calculation down into its parts, it's not quite as intimidating as it might seem.
Step 1 - Using your Beer's Law plots, calculate the iron concentration in the orange solutions you made in the second week of lab.
Step 2 - How did you make those orange solutions? You should have made them from 1.00mL of the yellow iron solution you made by dissolving your iron ore sample (red powder) in acid. You can use C1V1=C2V2 to determine the concentration of iron in the yellow solution.
Step 3 - Once you know the concentration (moles of iron per liter of solution) of the yellow iron solution, you can calculate how many moles of iron are present in that solution. You made 100.0mL of the yellow solution from your red iron ore sample.
Step 4 - If you know how many moles of iron were present in the yellow iron solution, you know how many moles of iron were present in the red iron ore powder. {Hint: they just might be the same...}
Step 5 - You know how many moles of iron were present in the red iron ore sample, using the Periodic Table you can calculate how many grams of iron were in the red iron ore sample.
Step 6 - If you know how many grams of iron were present in the red iron ore sample, and you know the total mass of the red iron or sample, then you should be able to calculate the percent (by mass) of iron that was present in the red iron ore sample.
For the iron ore samples you all analyzed, the percent iron could have been as low as 9-11% or as high as 33-35%. If you get something lower than 9% or higher than 35%, double check your calculations.
Iron Ore assignment
A couple notes on the Iron Ore hand-ins I've seen so far...
1. Include your Beer's Law plot(s). Your entire data analysis depends upon the Beer's Law plot(s) you made (probably in MSExcel) from the data you collected during the first week of the experiment. If you do not include your Beer's Law plot(s) {Absorbance vs. concentration}, you will not earn many points on this assignment.
2. Explain your answers. The answer to a question is (almost) never just "yes" or "no". Explain why "yes" or "no" is the correct answer. Does the sun come up in the east? Yes, because the rotation of the Earth causes the sun to be visible first in the direction humans have defined as "east".
3. Show calculations. This is related to #2. When you "show" calculations, it is helpful to include a little bit of explanation, not just a sea of numbers that appear to be scattered across the page. Similarly, DO NOT SHOW YOUR CALCULATIONS IN PARAGRAPH FORM! If you're looking for an example of "show calculations", check out some of the exam answer keys I have posted on my website {like http://www.drbodwin.com/teaching/exams/c210ke2ak.pdf}.
2012/10/30
This week's assignment...
I just looked over the assignments that have been handed in so far, and I have a couple additional notes:
1. For each combination that reacted during the first week of the experiment, write a balanced chemical equation. For reactions that formed complex ions, you can just note that the product of the reaction is “complex ion”. Organize your reactions clearly in the order they appear in the lab manual, starting with all the reactions involving Ni(NO3)2(aq), then all the reactions involving Pb(NO3)2(aq), etc.
>>If a combination did not react, you do not have to write a reaction. You can write "full formula/molecular equations" or net ionic equations, it's your choice for this experiment.
>>DO NOT WRITE YOUR CHEMICAL EQUATIONS AS ONE BIG PARAGRAPH!! The point of organizing them is to make them easier for you to write them all and for me to follow/read/grade them. If you write them as a massive block of text, I will not read them, and you probably won't earn many points. One equation per line, with appropriate subscripts and/or superscripts, state labels, parentheses, etc.
>>"in the order they appear in the lab manual" is not a joke or a suggestion. If you do not organize your chemical equations in the correct order, I will not spend much time hunting through your randomly-ordered equations to try to find them all. I will assume they are not there and you probably won't earn many points.
2. Identify the contents of each of your unknowns with a brief explanation. For example, “Unknown M4 contained phosphoric acid because it generated heat when reacting with bases and formed a precipitate with transition metal ions.”
>>Each set of unknowns had a letter AND a number associated with each sample. If you do not identify each unknown with a letter AND a number, it is wrong.
If you've already handed in an assignment and would like to fix some of these problems (that would be a really good idea...), you can turn in a revised assignment. If you choose to turn in a revised assignment, you must very clearly and obviously write or type "PLEASE GRADE THIS ASSIGNMENT AND DISCARD THE PREVIOUS ONE" on your assignment (you might even want to use a highlighter...). If you do not clearly and obviously indicate that you want the "new" assignment graded, I will grade the first assignment that was submitted.
1. For each combination that reacted during the first week of the experiment, write a balanced chemical equation. For reactions that formed complex ions, you can just note that the product of the reaction is “complex ion”. Organize your reactions clearly in the order they appear in the lab manual, starting with all the reactions involving Ni(NO3)2(aq), then all the reactions involving Pb(NO3)2(aq), etc.
>>If a combination did not react, you do not have to write a reaction. You can write "full formula/molecular equations" or net ionic equations, it's your choice for this experiment.
>>DO NOT WRITE YOUR CHEMICAL EQUATIONS AS ONE BIG PARAGRAPH!! The point of organizing them is to make them easier for you to write them all and for me to follow/read/grade them. If you write them as a massive block of text, I will not read them, and you probably won't earn many points. One equation per line, with appropriate subscripts and/or superscripts, state labels, parentheses, etc.
>>"in the order they appear in the lab manual" is not a joke or a suggestion. If you do not organize your chemical equations in the correct order, I will not spend much time hunting through your randomly-ordered equations to try to find them all. I will assume they are not there and you probably won't earn many points.
2. Identify the contents of each of your unknowns with a brief explanation. For example, “Unknown M4 contained phosphoric acid because it generated heat when reacting with bases and formed a precipitate with transition metal ions.”
>>Each set of unknowns had a letter AND a number associated with each sample. If you do not identify each unknown with a letter AND a number, it is wrong.
If you've already handed in an assignment and would like to fix some of these problems (that would be a really good idea...), you can turn in a revised assignment. If you choose to turn in a revised assignment, you must very clearly and obviously write or type "PLEASE GRADE THIS ASSIGNMENT AND DISCARD THE PREVIOUS ONE" on your assignment (you might even want to use a highlighter...). If you do not clearly and obviously indicate that you want the "new" assignment graded, I will grade the first assignment that was submitted.
2012/10/26
Improving your scores
A few people have express concern over their scores on hand-in assignments for lab. There's definitely room for improvement, but a single low score will not have that much impact on your grade, so keep at it. There are a couple general pointers that I've been giving students to help improve your hand-in scores:
1. Make sure you're answering the questions. I lot of people have been losing points because they're not really answering the question that's being asked, or they're answering part of it without really exploring WHY the answer they're giving is correct. Most of the questions on the hand-ins require a bit of explanation, make sure you're explaining your answers, not just describing an observation. If your answer takes up fewer lines of text than the question, you're probably not answering the question completely. That doesn't mean I want everyone to write a book to answer each question, but you should be trying to answer the whole question.
2. Don't lose points for mechanical reasons. Lots of people are "forgetting" to include graphs or data tables, or just showing a result without showing the calculations that went into that result. {No, your Lab Assistant did not tell you to turn your only copy of the graph in with your carbons...} Assignments must be typed (including formulas, equations, calculations, etc.), if you hand-write your assignment you'll probably get a zero. Circle or highlight your section/lab assistant so I can keep track of different piles of assignments or you'll lose 5 points. Assignments are due at very specific times listed in the course syllabus, repeated late or missing assignments will cost you a LOT of points and you'll very quickly fair the course. Grammar and spelling are important even though this isn't an English class; if your answer is very clear and easy to understand, it's easier to grade.
3. Take advantage of your time in lab. Many students finish the "wet" part of the experiment and then run out the door. If you finish early, make sure that you take a look at the hand-in assignment and discuss the questions and answers with your partner before you leave. If you're not sure about a question, ask.
4. Take advantage of your lab instructor(s). If you're working on a hand-in and you're not sure about what a question is asking or how to interpret your data, ask your instructor. This is more effective if you have a question that shows you put some thought into it, like "I started trying to analyze my data by calculating how many moles of each substance were used in each reaction, and then I calculated the mole fraction. What's the next step?", rather than "I don't understand how to do any of this. Where should I start?"
5. This is similar to #3... Start your assignment early so you have time to ask questions if necessary. If you wait until Tuesday night to work on an assignment that's due at noon on Wednesday, you'll never be able to ask a question and expect to get an answer in time to turn in the assignment on time.
Those are probably the big ones. If everyone did those things, everyone would be getting MUCH better scores on hand-in assignments. Something that students don't often know... It is MUCH MUCH MUCH easier to grade assignments that are correct! I don't ask questions with the intention to be sneaky, or tricky. It's not my goal to see how many points I can take away. I would be giggling with glee if everyone handed in assignments that were mostly (or totally) correct and complete. I really do want you to succeed, but that does not mean that I'm going to give you points or grades that you don't earn by doing the assignments and thinking about the questions you are answering.
1. Make sure you're answering the questions. I lot of people have been losing points because they're not really answering the question that's being asked, or they're answering part of it without really exploring WHY the answer they're giving is correct. Most of the questions on the hand-ins require a bit of explanation, make sure you're explaining your answers, not just describing an observation. If your answer takes up fewer lines of text than the question, you're probably not answering the question completely. That doesn't mean I want everyone to write a book to answer each question, but you should be trying to answer the whole question.
2. Don't lose points for mechanical reasons. Lots of people are "forgetting" to include graphs or data tables, or just showing a result without showing the calculations that went into that result. {No, your Lab Assistant did not tell you to turn your only copy of the graph in with your carbons...} Assignments must be typed (including formulas, equations, calculations, etc.), if you hand-write your assignment you'll probably get a zero. Circle or highlight your section/lab assistant so I can keep track of different piles of assignments or you'll lose 5 points. Assignments are due at very specific times listed in the course syllabus, repeated late or missing assignments will cost you a LOT of points and you'll very quickly fair the course. Grammar and spelling are important even though this isn't an English class; if your answer is very clear and easy to understand, it's easier to grade.
3. Take advantage of your time in lab. Many students finish the "wet" part of the experiment and then run out the door. If you finish early, make sure that you take a look at the hand-in assignment and discuss the questions and answers with your partner before you leave. If you're not sure about a question, ask.
4. Take advantage of your lab instructor(s). If you're working on a hand-in and you're not sure about what a question is asking or how to interpret your data, ask your instructor. This is more effective if you have a question that shows you put some thought into it, like "I started trying to analyze my data by calculating how many moles of each substance were used in each reaction, and then I calculated the mole fraction. What's the next step?", rather than "I don't understand how to do any of this. Where should I start?"
5. This is similar to #3... Start your assignment early so you have time to ask questions if necessary. If you wait until Tuesday night to work on an assignment that's due at noon on Wednesday, you'll never be able to ask a question and expect to get an answer in time to turn in the assignment on time.
Those are probably the big ones. If everyone did those things, everyone would be getting MUCH better scores on hand-in assignments. Something that students don't often know... It is MUCH MUCH MUCH easier to grade assignments that are correct! I don't ask questions with the intention to be sneaky, or tricky. It's not my goal to see how many points I can take away. I would be giggling with glee if everyone handed in assignments that were mostly (or totally) correct and complete. I really do want you to succeed, but that does not mean that I'm going to give you points or grades that you don't earn by doing the assignments and thinking about the questions you are answering.
2012/10/09
"Show your work" and "Explain"
In every field and for every instructor, there are certain instructions and questions that may not be explicitly stated, but are always implied. The two that I see people having the most consistent problems with are:
1. "Show your work" - If you are doing a calculation, you must ALWAYS show your work. If a question asks you to calculate the concentration of a solution, the answer is never just "1.18M". Show how you got to that number. You're doing it anyway, you should show it. If you're not doing it, then you're either "borrowing" an answer from someone else or guessing. Show your work. Sometimes, the way the question is worded might make it seem like you don't have to show your work ("What is the concentration of solution A?"), but you still need to show your work. The only real exception to "show your work" is for exceptionally trivial calculation, like adding up a molar mass (although you still should include the correctly balanced formula for anything you're adding up a molar mass for) or taking an average (if you really want to show the steps in taking an average, that's OK, but "average" is a standard enough operation that I don't require you to show your work). How do you decide if something is trivial or common enough that you don't have to show your work? One quick way to check is if the units have to be changed or transformed during the calculation. If the units change, show your work. When in doubt, show your work. If you're not sure, show your work. Even if you're really brilliant, show your work.
1a. "Show your work" = "Show your units" - This is another one that should be implied... In chemistry, there are a couple quantities that are unitless (pH, equilibrium constants), but the VAST majority of numbers should include units. If you do not show your units, you have not shown your work. Always show units on every number that has them. THE ONLY EXCEPTION is in tabulated data where the units can (and often should) be included in the column heading of the table rather than on each individual number.
2. "Explain" - If science was only used to answer the "what" aspects of our daily lives, it would be pretty boring. Science answers the "what" AND the "why". The "why" is always implied. This is the text analog of "show your work". I do not tend to ask questions that I feel can be answered in 1 word or a short phrase. I want to see an explanation of why the answer you gave makes sense in relation to the data you have collected. This does not mean that you have to write a page and a half to answer every question, but it does mean that a sentence or two (or more) is probably required. Support your answers with meaningful data or other explanations, it makes for a much clearer answer.
1. "Show your work" - If you are doing a calculation, you must ALWAYS show your work. If a question asks you to calculate the concentration of a solution, the answer is never just "1.18M". Show how you got to that number. You're doing it anyway, you should show it. If you're not doing it, then you're either "borrowing" an answer from someone else or guessing. Show your work. Sometimes, the way the question is worded might make it seem like you don't have to show your work ("What is the concentration of solution A?"), but you still need to show your work. The only real exception to "show your work" is for exceptionally trivial calculation, like adding up a molar mass (although you still should include the correctly balanced formula for anything you're adding up a molar mass for) or taking an average (if you really want to show the steps in taking an average, that's OK, but "average" is a standard enough operation that I don't require you to show your work). How do you decide if something is trivial or common enough that you don't have to show your work? One quick way to check is if the units have to be changed or transformed during the calculation. If the units change, show your work. When in doubt, show your work. If you're not sure, show your work. Even if you're really brilliant, show your work.
1a. "Show your work" = "Show your units" - This is another one that should be implied... In chemistry, there are a couple quantities that are unitless (pH, equilibrium constants), but the VAST majority of numbers should include units. If you do not show your units, you have not shown your work. Always show units on every number that has them. THE ONLY EXCEPTION is in tabulated data where the units can (and often should) be included in the column heading of the table rather than on each individual number.
2. "Explain" - If science was only used to answer the "what" aspects of our daily lives, it would be pretty boring. Science answers the "what" AND the "why". The "why" is always implied. This is the text analog of "show your work". I do not tend to ask questions that I feel can be answered in 1 word or a short phrase. I want to see an explanation of why the answer you gave makes sense in relation to the data you have collected. This does not mean that you have to write a page and a half to answer every question, but it does mean that a sentence or two (or more) is probably required. Support your answers with meaningful data or other explanations, it makes for a much clearer answer.
2012/09/29
Aluminum + HCl experiment assignment
A few people have asked questions that lead me to believe that there may be a little confusion about the assignment that is due for "The Reaction of Aluminum with a Strong Acid".
This semester in Gen Chem Lab I, you will not have to write a full lab report for any of the experiments. Each experiment will have a hand-in assignment provided in MSWord format with questions that you will have to answer. {All answers and calculations must be typed. No exceptions.} For a number of these assignments, there will be a part of a full lab report included as one of the questions, but you will not have to do all the parts of a formal lab report for any single experiment.
The questions that are included in the lab procedure (either inserted in the experiment or at the end of the procedure) do not have to be included or handed in separately, the only things that have to be turned in are your carbons and the hand-in assignment. The questions in the experimental procedure are often similar to the questions on the hand-in, so it's a good idea to jot down some answers to the questions in the procedure while you're in lab, but these do not have to be turned in separately.
To help with some of the questions that might come up, I've posted "Features of a Sample Lab Report Format" (http://www.drbodwin.com/teaching/genchemlab/labrep01.pdf) so you can see what each part of a lab report looks like when you need it. A couple questions or problems that come up are...
Drawing the experimental setup - A drawing can be helpful for complex setups that are difficult to describe well with words. A drawing is a waste of space if the setup can be described effectively and concisely with words. For example: "The test tube was filled approximately half full with the mysterious blue liquid, see figure 1."
Hopefully this is ridiculous enough to make my point... Is the figure really needed to clearly communicate what a test tube half full of blue liquid looks like? Um, no. The same is true for your "Experimental" section when you eventually have to write one. Don't draw trivial things unless your intent is to be trivial, which should never be your intent in a lab report.
Can I hand draw/write in by hand? - This question often comes up regarding drawing experimental setups and showing sample calculations. The quick answer is... no. Why not? There are a few reasons, but the most important one is for your own reference. If you just leave a big blank space for a hand-drawn experimental setup and then "the dog ate my homework" happens before you turn it in, you have to redraw everything rather than just re-print. Similarly, if later in the semester or year you want to look back at an experiment but you can't find the paper copy that you turned in, opening up the computer file and seeing a big blank box isn't too helpful. This is even more important with calculations, if you write out 2 pages of carefully thought out sample calculations and then spill your coffee (or juice or lasagne) on them, you'll have to re-think and re-write 2 pages of calculations. Just do everything electronically in the first place and at least 4 things will happen: 1) you'll have nice looking assignments; 2) you'll actually catch a few errors you might have missed; 3) you'll never have to re-write a page that gets lost or destroyed; 4) with a little practice, you'll get good at doing all these things electronically. Speaking of calculations...
Do I have to use one of those equation editor thingies or can I just use tabs and spaces to type my sample calculations? - Use an equation editor. They're not that hard to use (with a little practice) and they can make really nice looking equations. If the sample calculation you're trying to show is very simple, it's OK to just type it in-line, for example:
There are a couple good equation editor options. MSWord has at least one (sometimes 2) built in, Open Office has a good one built in, and the Daum Equation Editor (I got it in the Chrome web store...) also does a good job. As with any software, practice makes these things easier to use.
Good luck and let me know if there are any other questions.
This semester in Gen Chem Lab I, you will not have to write a full lab report for any of the experiments. Each experiment will have a hand-in assignment provided in MSWord format with questions that you will have to answer. {All answers and calculations must be typed. No exceptions.} For a number of these assignments, there will be a part of a full lab report included as one of the questions, but you will not have to do all the parts of a formal lab report for any single experiment.
The questions that are included in the lab procedure (either inserted in the experiment or at the end of the procedure) do not have to be included or handed in separately, the only things that have to be turned in are your carbons and the hand-in assignment. The questions in the experimental procedure are often similar to the questions on the hand-in, so it's a good idea to jot down some answers to the questions in the procedure while you're in lab, but these do not have to be turned in separately.
To help with some of the questions that might come up, I've posted "Features of a Sample Lab Report Format" (http://www.drbodwin.com/teaching/genchemlab/labrep01.pdf) so you can see what each part of a lab report looks like when you need it. A couple questions or problems that come up are...
Drawing the experimental setup - A drawing can be helpful for complex setups that are difficult to describe well with words. A drawing is a waste of space if the setup can be described effectively and concisely with words. For example: "The test tube was filled approximately half full with the mysterious blue liquid, see figure 1."
 |
| Figure 1: A half full test tube |
Can I hand draw/write in by hand? - This question often comes up regarding drawing experimental setups and showing sample calculations. The quick answer is... no. Why not? There are a few reasons, but the most important one is for your own reference. If you just leave a big blank space for a hand-drawn experimental setup and then "the dog ate my homework" happens before you turn it in, you have to redraw everything rather than just re-print. Similarly, if later in the semester or year you want to look back at an experiment but you can't find the paper copy that you turned in, opening up the computer file and seeing a big blank box isn't too helpful. This is even more important with calculations, if you write out 2 pages of carefully thought out sample calculations and then spill your coffee (or juice or lasagne) on them, you'll have to re-think and re-write 2 pages of calculations. Just do everything electronically in the first place and at least 4 things will happen: 1) you'll have nice looking assignments; 2) you'll actually catch a few errors you might have missed; 3) you'll never have to re-write a page that gets lost or destroyed; 4) with a little practice, you'll get good at doing all these things electronically. Speaking of calculations...
Do I have to use one of those equation editor thingies or can I just use tabs and spaces to type my sample calculations? - Use an equation editor. They're not that hard to use (with a little practice) and they can make really nice looking equations. If the sample calculation you're trying to show is very simple, it's OK to just type it in-line, for example:
7.3 + 2.9 + 14.82 + 1.4 = 26.4
There's no reason to use an equation editor for that one, it's perfectly clear and clean typed in-line. On the other hand...
(2.635g Cu)(1 mole Cu / 63.546g Cu)(4 moles NH3 / 1 mole Cu)(1L NH3(aq) / 2.83 moles NH3) = 0.0586L NH3(aq)
Looks horrible and is hard to follow when typed in-line, it would look much better if an equation editor was used:
Good luck and let me know if there are any other questions.
2012/09/26
Posting and removing info
A few people have had some questions about info that's posted in D2L, so...
Removing files - The hand-in files will not be removed from D2L before lab. It would usually be a good idea for you to download and read over the hand-in before your lab, but it will be available. I do remove the experimental procedures for 2 reasons:
1. To encourage everyone to read over the experimental procedure before coming to lab, and maybe even take a couple notes on it when you're watching the pre-lab video
2. To discourage people from printing out the experimental procedure over and over and over again. Poor little trees died to make all that paper, I always feel bad when I throw piles of excess printouts in the recycle bins...
Files not opening - D2L is a very helpful system, but sometimes it gets a little grumpy. With the exception of the quizzes, everything that's in D2L is just a link to something that's on my webpage (http://www.drbodwin.com/teaching/genchemlab.php). If a D2L link isn't working, pop over to my webpage and you should be able to find it there. If there's a broken link on my webpage, let me know so I can fix it.
Those were the two big questions that people have had. If there are others, let me know. Good luck in lab this week.
Removing files - The hand-in files will not be removed from D2L before lab. It would usually be a good idea for you to download and read over the hand-in before your lab, but it will be available. I do remove the experimental procedures for 2 reasons:
1. To encourage everyone to read over the experimental procedure before coming to lab, and maybe even take a couple notes on it when you're watching the pre-lab video
2. To discourage people from printing out the experimental procedure over and over and over again. Poor little trees died to make all that paper, I always feel bad when I throw piles of excess printouts in the recycle bins...
Files not opening - D2L is a very helpful system, but sometimes it gets a little grumpy. With the exception of the quizzes, everything that's in D2L is just a link to something that's on my webpage (http://www.drbodwin.com/teaching/genchemlab.php). If a D2L link isn't working, pop over to my webpage and you should be able to find it there. If there's a broken link on my webpage, let me know so I can fix it.
Those were the two big questions that people have had. If there are others, let me know. Good luck in lab this week.
2012/09/11
Still some confusion...
Hmm, I think a few people are still confused about what is due this week. Let me try to be very clear so that we all (hopefully) get the same message:
The assignment that is due this week by Wednesday at noon is:
http://www.drbodwin.com/teaching/genchemlab/c150L2012d01DandE1hi1.docx
It's a MSWord document. It needs to be typed. It should be printed and turned in to the assignment box for your lab section located outside HA103. Make sure you indicate your section/room/Lab Assistant. This is the hand-in assignment that is based upon the Data and Error activity we did in lab last week. You do not have to turn in anything else, your lab notebook carbons were turned in before you left lab last week.
There is also a pre-lab quiz due by Wednesday noon. It covers the MSExcel activity that we will be doing this week in lab as well as the safety info you should be looking at.
http://www.drbodwin.com/teaching/genchemlab/c150L2012d02excel1.pdf
http://www.drbodwin.com/teaching/genchemlab/safetyslides01a.pdf
http://www.drbodwin.com/teaching/genchemlab/safetyslides02a.pdf
OK, that might have sounded a little blunt. I wasn't trying to sound grumpy, I just wanted to be as clear as possible to (again, hopefully) avoid any confusion. I know these first couple weeks of lab can be a little busy because we're doing a bunch of different things to get caught up on safety and general lab issues, next week we'll be starting our first "wet" experiment. And yes, it will have the potential to get very wet!
The assignment that is due this week by Wednesday at noon is:
http://www.drbodwin.com/teaching/genchemlab/c150L2012d01DandE1hi1.docx
It's a MSWord document. It needs to be typed. It should be printed and turned in to the assignment box for your lab section located outside HA103. Make sure you indicate your section/room/Lab Assistant. This is the hand-in assignment that is based upon the Data and Error activity we did in lab last week. You do not have to turn in anything else, your lab notebook carbons were turned in before you left lab last week.
There is also a pre-lab quiz due by Wednesday noon. It covers the MSExcel activity that we will be doing this week in lab as well as the safety info you should be looking at.
http://www.drbodwin.com/teaching/genchemlab/c150L2012d02excel1.pdf
http://www.drbodwin.com/teaching/genchemlab/safetyslides01a.pdf
http://www.drbodwin.com/teaching/genchemlab/safetyslides02a.pdf
OK, that might have sounded a little blunt. I wasn't trying to sound grumpy, I just wanted to be as clear as possible to (again, hopefully) avoid any confusion. I know these first couple weeks of lab can be a little busy because we're doing a bunch of different things to get caught up on safety and general lab issues, next week we'll be starting our first "wet" experiment. And yes, it will have the potential to get very wet!
2012/09/04
First Lab
CHEM 150L labs will start this week. There have been a few questions, I'll try to answer all of them here:
1. Pre-lab Quiz - As of this morning, there are quite a few people who have not taken the pre-lab quiz. This first quiz is largely on the syllabus and the first week's Data and Error activity. Read these, have them handy, and make sure you take the quiz. Read over the part of the syllabus that describes pre-lab quizzes and the results of not taking them. Skipping quizzes will have a very quick and very negative impact on your grade.
2. A number of people have asked about what to bring to lab. I've moved/copied a couple of things into the "Week 1" content section in D2L that should help... You should print out this week's activity ("Data and Error"), the "Safety Map", and the two "Video Guides". Make sure you print these out BEFORE LAB. To discourage repeated and excessive printing, these links will not be available after NOON WEDNESDAYS. Also, bring your "carbon copy" lab notebook to lab with youand a pen.
2a. If you're a eBook/eReader user, feel free to use electronic versions of the lab procedures on your personal devices. If you choose to use a tablet or eReader in lab, be careful, there are numerous opportunities to spill things that will turn your device into an expensive paperweight.
2b. If you choose to print procedures out on paper, I would encourage you double-side or use 2-pages-per-sheet format.
3. Watch the "Safety Tour (video)". It goes over some of the safety features and/or equipment in the labs you will be in.
4. IF YOU ARE IN DR. MARASINGHE'S CLASS (Wed 2:30pm or Thurs 3:00pm), your class will meet in SL118 to begin class before moving up to the labs (SL302 and SL306)
5. IF YOU ARE IN DR. PROVOST'S CLASS (Thurs 9:00am) OR DR. EDVENSON'S CLASS (Thurs noon), your class will meet in the hallway outside of SL302 and SL306 for a minute before going directly into the labs.
6. Proper Lab Attire - There will be more details on this during some of the safety discussions that we have during the first couple weeks in lab, but there are a couple things to remember: 1) Open-toed shoes are not allowed in lab. The best option for safe lab footwear is probably something like leather (durable and liquids don't soak through), but as long as your footwear is closed-toe it should be OK. 2) Lab coats or aprons are not required, but you are welcome to wear them if you choose.
If there are other questions, let me know. Have a great first week in lab!
1. Pre-lab Quiz - As of this morning, there are quite a few people who have not taken the pre-lab quiz. This first quiz is largely on the syllabus and the first week's Data and Error activity. Read these, have them handy, and make sure you take the quiz. Read over the part of the syllabus that describes pre-lab quizzes and the results of not taking them. Skipping quizzes will have a very quick and very negative impact on your grade.
2. A number of people have asked about what to bring to lab. I've moved/copied a couple of things into the "Week 1" content section in D2L that should help... You should print out this week's activity ("Data and Error"), the "Safety Map", and the two "Video Guides". Make sure you print these out BEFORE LAB. To discourage repeated and excessive printing, these links will not be available after NOON WEDNESDAYS. Also, bring your "carbon copy" lab notebook to lab with youand a pen.
2a. If you're a eBook/eReader user, feel free to use electronic versions of the lab procedures on your personal devices. If you choose to use a tablet or eReader in lab, be careful, there are numerous opportunities to spill things that will turn your device into an expensive paperweight.
2b. If you choose to print procedures out on paper, I would encourage you double-side or use 2-pages-per-sheet format.
3. Watch the "Safety Tour (video)". It goes over some of the safety features and/or equipment in the labs you will be in.
4. IF YOU ARE IN DR. MARASINGHE'S CLASS (Wed 2:30pm or Thurs 3:00pm), your class will meet in SL118 to begin class before moving up to the labs (SL302 and SL306)
5. IF YOU ARE IN DR. PROVOST'S CLASS (Thurs 9:00am) OR DR. EDVENSON'S CLASS (Thurs noon), your class will meet in the hallway outside of SL302 and SL306 for a minute before going directly into the labs.
6. Proper Lab Attire - There will be more details on this during some of the safety discussions that we have during the first couple weeks in lab, but there are a couple things to remember: 1) Open-toed shoes are not allowed in lab. The best option for safe lab footwear is probably something like leather (durable and liquids don't soak through), but as long as your footwear is closed-toe it should be OK. 2) Lab coats or aprons are not required, but you are welcome to wear them if you choose.
If there are other questions, let me know. Have a great first week in lab!
2012/08/28
CHEM 150L - Fall 2012
Lab info is posted in D2L including your first quiz. Lab will not meet this week, but you should read the syllabus and the activity that we're going to be doing next week before you do the quiz. Good luck and welcome to Fall 2012!
2012/07/26
Summer 2012 grades posted
Hmm, not sure I need to say more than what's in the title of the post. I submitted grades to the eServices system, they'll be visible to you whenever all the correct electrons flow through all the correct wires to make the interwebs update to display your grades.
I hope everyone had a useful class this summer, let me know if you have any questions and enjoy the rest of your summer.
I hope everyone had a useful class this summer, let me know if you have any questions and enjoy the rest of your summer.
2012/07/24
Corrections to Exam 4 key
Typing up the key late at night was not a good idea... There were a number of errors on page 4 of the Exam 4 key that I posted this morning. If you already looked at the key or printed it out, throw away page 4 and go get a fresh version. Sorry about that. http://www.drbodwin.com/teaching/exams/c210pe4ak.pdf
Exams posted
All the exams we've taken for the summer 2012 class and their keys are posted on my web page, both on the "General Chemistry" page (http://www.drbodwin.com/teaching/genchem.php) and in the "Exam Archive" (http://www.drbodwin.com/teaching/examarchive.php). Let me know if you have questions.
2012/07/23
Redox reactions
We looked at a few redox reactions
today in class, some of you wanted the solutions posted. Enjoy.
For each pair of half cells, write the balanced
spontaneous (standard) reaction and calculate the spontaneous
(standard) cell voltage. {Actually, you'd do the second
part of that process before you could do the first part}
Cu+1|Cu
(E0red
= +0.521V) and Fe3+|Fe2+
(E0red
= +0.771V)
Cu
half must be reversed to be the oxidation half rxn
E0cell
= E0red
+ E0ox
= 0.771V + (-0.521V) = +0.250V
Cu(s)
⇄ Cu+1(aq)
+ 1e⊖(aq)
1e⊖
+ Fe3+(aq)
⇄ Fe2+(aq)
------------------------------------------------------------------
Cu(s)
+ Fe3+(aq)
⇄ Fe2+(aq)
+ Cu+1(aq)
BrO3-1|Br2
(E0red
= +1.478V) and Pt2+|Pt
(E0red
= +1.188V)
Pt
half must be reverse to be the oxidation half rxn
E0cell
= E0red
+ E0ox
= 1.478V + (-1.188V) = +0.290V
5
{ Pt(s) ⇄ Pt+2(aq)
+ 2e⊖(aq)
}
12
H⊕(aq)
+ 10e⊖(aq)
+ 2 BrO3-1(aq)
⇄ Br2(l)
+ 6 H2O(l)
------------------------------------------------------------------
12
H⊕(aq)
+ 5 Pt(s) + 2 BrO3-1(aq)
⇄ Br2(l)
+ 5 Pt+2(aq)
+ 6 H2O(l)
Cr+2|Cr
(E0red
= -0.403V) and Pb+4|Pb+2
(E0red
= +1.69V)
Cr
half must be reverse to be the oxidation half rxn
E0cell
= E0red
+ E0ox
= 1.69V + (0.403V) = +2.09V
Cr(s)
⇄ Cr+2(aq)
+ 2e⊖(aq)
2e⊖
+ Pb+4(aq)
⇄ Pb+2(aq)
------------------------------------------------------------------
Cr(s)
+ Pb+4(aq)
⇄ Pb2+(aq)
+ Cr+2(aq)
TeO2|Te
(E0red
= +0.604V) and Se|H2Se
(E0red
= -0.115V)
Se
half must be reverse to be the oxidation half rxn
E0cell
= E0red
+ E0ox
= 0.604V + (0.115V) = +0.719V
2
{ H2Se(aq)
⇄ Se(s) + 2e⊖(aq)
+ 2 H⊕(aq)
}
4
H⊕(aq)
+ 4e⊖
+ TeO2(s)
⇄ Te(s) + 2 H2O(l)
------------------------------------------------------------------
2
H2Se(aq)
+ TeO2(s)
⇄ Te(s) + 2 Se(s) + 2 H2O(l)
MnO4-1|Mn+2
(E0red
= +1.23V) and ClO4-1|ClO3-1
(E0red
= +1.201V)
Cl
half must be reverse to be the oxidation half rxn
E0cell
= E0red
+ E0ox
= 1.23V + (-1.201V) = +0.03V
5
{ H2O(l)
+ ClO3-1(aq)
⇄ ClO4-1(aq)
+ 2e⊖(aq)
+ 2 H⊕(aq)
}
2
{ 8 H⊕(aq)
+ 5e⊖
+ MnO4-1(aq)
⇄ Mn2+(aq)
+ 4 H2O(l)
}
------------------------------------------------------------------
616
H⊕(aq)
+ 5 H2O(l)
+ 2 MnO4-1(aq)
+ 5 ClO3-1(aq)
⇄ 5 ClO4-1(aq)
+ 2 Mn2+(aq)
+ 38
H2O(l)
+ 10 H⊕(aq)
6
H⊕(aq)
+ 5 H2O(l)
+ 2 MnO4-1(aq)
+ 5 ClO3-1(aq)
⇄ 5 ClO4-1(aq)
+ 2 Mn2+(aq)
+ 3 H2O(l)
2012/07/12
Titrations are AWESOME!!
We looked at titrations today.
Remember, titrations are just stoichiometry problems applied to a
specific system/type of problem, they're not completely new
information, approach them the same way you would approach any other
stoichiometry problem:
1. Write a balanced chemical equation
2. Convert whatever you know the most about to
moles
3. Using the mole ratio from the balanced
chemical equation, convert moles of what you know to moles of what
you're looking for
4. Convert moles of what you're looking for
into whatever you want to know about it (grams, volume,
concentration, etc.)
5. Check that your answer is reasonable (if
possible)
On to today's problems...
30.00mL of 0.713M HNO2(aq)
is titrated to the equivalence point with 28.43mL of NaOH(aq) of an
unknown concentration. What is the concentration of the NaOH(aq)
stock solution? What was the pH of the HNO2(aq) solution
before the titration begins? What is the pH at the equivalence point?
{Ka(HNO2) = 4.0x10-4}
HNO2(aq)
+ NaOH(aq) ⇄ H2O(l) + NaNO2(aq)
(0.03000L
HNO2(aq)) (0.713M HNO2(aq)) = 0.02139mols
HNO2
(0.02139mols
HNO2)
(1mol NaOH / 1mol HNO2)
= 0.02139mols NaOH
(0.02139mols
NaOH) / (0.02843L NaOH(aq)) = 0.752M NaOH(aq)
NaOH(aq)
should be slightly more concentrated than HNO2(aq), so
this answer is reasonable
Before the titration begins, this is an
aqueous solution of a weak acid, so we can calculate the pH using a
Ka-type approach. Setting up a table...
-
HNO2(aq) +H2O(l) ⇄H3O+(aq) +NO2-1(aq)[ ]initial0.713MXXXX00Δ[ ]- xXXXX+ x+ x[ ]equilibrium(0.713 – x) MXXXXx Mx M
Assuming that “x” is
much less than 0.713, the Ka expression simplifies to:
Ka = (x)(x) /
(0.713) = 4.0x10-4
x = 0.01689 = [H3O+]
pH = -log[H3O+]
= -log(0.01689) = 1.77
At the equivalence point, all of the
HNO2(aq) that was originally in the reaction has reacted
with OH-1(aq) to form nitrite ions, NO2-1(aq).
The titration started with:
(0.03000L
HNO2(aq)) (0.713M HNO2(aq)) = 0.02139mols
HNO2
So at the equivalence point we have a
solution that contains 0.02139mols of NO2-1(aq)
in (30.00mL + 28.43mL = 58.43mL) of solution. The concentration of
NO2-1(aq) at the equivalence point is:
(0.02139mols of NO2-1(aq))
/ (0.05843L) = 0.3659M NO2-1(aq)
This can now be plugged in to a Kb-type
equilibrium to solve...
-
NO2-1(aq) +H2O(l) ⇄OH-1(aq) +HNO2(aq)[ ]initial0.3659MXXXX00Δ[ ]- xXXXX+ x+ x[ ]equilibrium(0.3659 – x) MXXXXx Mx M
Assuming that “x” is
much less than 0.3659, the Kb expression simplifies to:
Kb = (x)(x) /
(0.3659) = 2.5x10-11
x = 3.02x10-6
= [OH-1]
pOH = -log[OH-1]
= -log(3.02x10-6) = 5.519
pH = 14 – 5.519 =
8.48
15.00mL of sulfurous acid of
unknown concentration is titrated to the second equivalence
point with 23.18mL of 0.332M NaOH(aq). What is the concentration of
the sulfurous acid stock solution?
H2SO3(aq)
+ 2 NaOH(aq) ⇄ H2O(l) + Na2SO3(aq)
(0.02318L
NaOH(aq)) (0.332M NaOH(aq)) = 7.696x10-3mols NaOH
(7.696x10-3mols
NaOH) (1mol H2SO3
/ 2mol NaOH) = 3.848x10-3mols
H2SO3
(3.848x10-3mols
H2SO3)
/ (0.01500L H2SO3(aq)) = 0.257M H2SO3(aq)
NaOH(aq)
should be slightly more concentrated than H2SO3(aq),
so this answer is reasonable
I'll be in tomorrow morning, let me
know if you have any questions.
2012/07/11
Crunching through K_a problems...
4. What
is the expected pH of a 2.49M solution of acetic acid {Ka
= 1.8x10-5}? What is the expected pH when 100.0mL of this
solution is combined with 100.0mL of water? Assume volumes are
additive.
This
is a Ka-type equilibrium problem, organize the information
using a table.
- HC2H3O2(aq) +H2O(l) ⇄H3O+(aq) +C2H3O2-1(aq)[ ]initial2.49 MXXXX0 M0 MΔ[ ]- xXXXX+ x+ x[ ]equilibrium(2.49 – x) MXXXXx Mx M
Assume
“x” is much smaller than 2.49, plug in to the equilibrium
constant expression...
x
= 6.69x10-3 = [H3O+1]
Assumption
is good.
pH
= -log[H3O+1] = -log(6.69x10-3)
= 2.174
For
the second part, the set-up is the same, the only difference is that
the initial concentration of acetic acid has been diluted.
Calculating the dilution...
C1V1
= C2V2
(2.49M)(100.0mL)
= C2(200.0mL)
C2
= 1.245M
Plug
in and solve the same way:
x
= 4.73x10-3 = [H3O+1]
Assumption
is still good.
pH
= -log[H3O+1] = -log(4.73x10-3)
= 2.325
5. A
1.83M solution of a weak, monoprotic acid {HA(aq)} has a pH of 3.48.
What is the Ka of this acid?
We
can approach this as a Ka-type equilibrium problem as
well, organize the information using a table.
- HA(aq) +H2O(l) ⇄H3O+(aq) +A-1(aq)[ ]initial1.83 MXXXX0 M0 MΔ[ ]- xXXXX+ x+ x[ ]equilibrium(1.83 – x) MXXXXx Mx M
In
this case, we are given a pH, which gives us a way to calculate
[H3O+], which gives us “x”...
[H3O+]
= 10-pH = 10-3.48 = 3.3113x10-4
(Assumption is good.)
Ka
= (3.3113x10-4)(3.3113x10-4) / (1.83) =
5.99x10-8
6. You
have combined 100.0mL of 2.84M hydrofluoric acid {Ka =
6.8x10-4} and 100.0mL of 2.19M fluoride ions. What is the
expected pH of the resulting solution? Assume volumes are additive.
Again,
we can approach this as a Ka-type equilibrium problem
(Noticing a pattern here?), organize the information using a table.
- HF(aq) +H2O(l) ⇄H3O+(aq) +F-1(aq)[ ]initial1.42 MXXXX0 M1.095 MΔ[ ]- xXXXX+ x+ x[ ]equilibrium(1.42 – x) MXXXXx M(1.095 + x) M
A
couple little adjustments in this case... the original concentrations
given in the problem have to be diluted to get the “initial”
concentrations in the table. The other key difference here is that
we're starting out with a mixture of reactants and products. That
might clutter up the math a little bit, but it doesn't really change
the way we approach the problem. To simplify things, let's assume
that “x” is small compared to both 1.42 and 1.095. Then
the Ka expression is:
6.8x10-4
= (x)(1.095) / (1.42)
x
= 8.818x10-4
Assumption
is good.
pH
= -log[H3O+1] = -log(8.818x10-4)
= 3.055
Subscribe to:
Posts (Atom)
