Titrations are stoichiometry problems

Today we talked about titrations. Titrations are (usually) used to determine the concentration of an acid or base in solution, although they can be used in a LOT of places other than acid-base reactions. Treat them the same way you approach all stoichiometry problems.

The extra SI session next week will be next Monday at 6:30 in BR269. We have an exam next Wednesday.

Catch up...

Yikes, it's been a while...

Since the last exam, we've been looking at stoichiometry and various classes and types of reactions. This is chapter 4 & 5 material from the textbook. The heart of every stoichiometry problem is the mol-to-mol conversion made using the coefficients from the balanced chemical equation. The rest of the problem is all about getting into and out of moles.

We're currently in the middle of reaction types, we've talked about precipitations and molecule-forming reactions so far. We'll finish up acids and bases on Monday and move on to everyone's favorite reaction type, redox!

Have a good weekend, the weather's supposed to be nice. If you need a study break, the soccer team is at home this weekend on both Saturday and Sunday at 1pm. What could be better than a beautiful fall day and a little soccer?

Another question...

From email...
---
Just wondering if you could tell me what I am doing wrong for #8 on last years chem 150 test. The question is "What is the formula weight of nickel(II) nitrate?"
Heres what I did:
Nickel is 58.69 and there are 2 so I took (2)58.69. Nitrate is NO3 so Nitrogen is 14.01 and Oxygen is 16.00 and there are 3 so it would be 14.01+3(16). This leads me to:
2(58.69)+14.01+3(16) which gives me 179.39. This is the wrong answer...on the answer key the answer is 182.70. Just wondering what I did wrong. Thanks!
---
A lot of people trip up on this one. Remember, when there's a roman numeral after a metal, that tells you the charge of the metal cation, it does not tell you how many of that cation are in the balanced formula. For this one, the nickel has a +2 charge. Nitrate has a charge of -1, so to balance the charge of the formula, we need two NO₃^-1 for each Ni⁺², Ni(NO₃)₂, so the formula weight of nickel(II) nitrate should be:
(58.69g/mol) + 2(14.007g/mol) + 6(15.999g/mol) = 182.70g/mol

Other questions, let me know...

Exam questions...

Already a couple questions...

---Question----------------------------------------------------------------

Is the empirical formula just the smallest whole number ratio? What if it came out C=1.5 H=2.5 O=3 or something like that? Is that still the empirical formula? or would the empirical formula be C=3 H=5 O=6, and then work from there to get your molecular formulas?!

---Answer----------------------------------------------------------------

Yes, the empirical formula is the smallest whole number ratio, so in your example the most correct way to report the empirical formula would be C₃H₅O₆. The molecular formula would be some multiple of that and you'd have to be given more information in the problem to determine the correct molecular formula.

---Question----------------------------------------------------------------

For percent composition questions with multiple elements, will be expected to have the elements in the correct order in the final answer? For example: KMnO₄ instead of say MnKO₄....Or will the main concern be that we achieved the correct amount of each element?

---Answer----------------------------------------------------------------

Quite a few people have asked me about this and in general the order doesn't matter. The only place your should pay attention to the order and groupings is in the formulas of ionic compounds and polyatomic ions. When writing the formula for an ionic compound it is usually best accepted practice to list the cation first, followed by the anion, and you should always write polyatomic ions as their common formula is written. In your example, since permanganate is a polyatomic ion, it should always be written together as "MnO₄^-". Since this is an ionic compound, the cation {potassium ion} should also be written first, so this should be written KMnO₄. That's not a result of it being a percent composition problem, that's the naming convention for ionic compounds.

If you have other questions, let me know, I'll post answers to the blog ASAP.

Suggested problems from the text...

I've posted some suggested problems from the textbook on my web page. These are not required problems, they will not be collected, they will not be graded, they are merely problems that I think are good ones from the end of each chapter if you'd like some additional practice.

Almost exam time...

Sara was unable to get a room, so there will not be an extra SI session tonight.

Next Monday is your first exam, be sure to look at the old exams on my webpage to get an idea of what to expect. Also, make sure that if you intend to use a calculator on the exam it cannot be a graphing/programmable calculator, or a cell phone calculator, or an iPod/iPad calculator, or any other networked or interactive "calculator experience". Sit with at least 1 open chair between everyone.

Today we worked through another percent composition question, this time using percent composition to determine the formula of a salt that someone carelessly neglected to label completely.

If you have questions you'd like to go over in class on Friday, let me know in advance or just bring them to class and we'll go through as many as we have time for.

The cusp of the Labor Day weekend...

There is (was?) a problem with the time in OWL, it has been set to east coast time. I've contacted a few people at OWL about this and it should be fixed soon if it's not already corrected. If it's not corrected and you happen to be doing your OWL assignment between 11:00pm and 11:59pm, please continue to do the assignment even if you get a "past due" message. When the clock is corrected, the system will automatically re-grade your assignment.

There's new OWL posted, due next Friday.

Lab hand-ins turned in by 3:00pm on Mondays get a bonus point, BUT next Monday is Labor Day, so the university will not be open (at least not completely). Therefore, for this assignment only you will get a bonus point as long as your assignment is in by noon Tuesday. The regular deadline, Wednesday at 3:00pm, is still in place.

Today in class we looked at ionic formulas again and checked out how to calculate percent composition from a chemical formula, and a chemical formula from percent composition. One question a few people had about the example I did was "Where did that 78g/mol number come from?" That's a number that would have to be given in the problem for you to use, it's not something you would calculate in that specific example.

If you have any input about the SI schedule, please let Sara know. If none of the current times fit in your schedule, or if some other time would work better in your schedule, she is open to adjusting the SI times to better serve as many student as possible. Keep in mind that Sara is a student, so she also has a lot of time commitments for classes and study time and other activities (and she probably likes to sleep an hour or two every night...).

Enjoy your 3-day weekend, be safe and I'll see you on Wednesday.

Announcements and ionic compounds

Today we had LOTS of announcements, so:

Dr. Wallert announced the bi-weekly (that's every other week, not twice a week...) Biochemistry and Biotechnology Seminar Series. There are some very interesting talks and topics planned, so keep them in mind. There are poster hanging around the building, look for the ear of corn with some funny looking kernels for more details.

Tonight is the first meeting of the Chemistry and Biochemistry Club. SL104, 7pm. CBC is a great opportunity to get involved on campus and in the community, and gives you an opportunity to get to know other chemistry and biochemistry majors. There will be pizza and soda...

Keep an eye on the schedule, our first exam is coming up on September 13th...that's less than 2 weeks away. It will cover Chapters 1-3. Don't forget about the calculator rules: no graphing calculators, no sharing calculators, no cell phones, no iPods/iPads. If it's more complex than a basic $15 scientific calculator, it's probably not allowed. If you're not sure, check with me BEFORE the exam.

Today in class we looked at ionic compounds. You will be required to know any polyatomic ion listed in Table 3.7, page 88 of the textbook.