Email questions...

There are a bunch, I'll answer them one at a time...

I have some questions on the old exams. One is balancing equations. I know we've talked about that alot in class, but I'm still kind of confused. I understand how to balance the numbers out when we're given the equation, but I get confused when starting from scratch. For example, number 7 for fall 2006 exam: Magnesium hydroxide solution + Lead (IV) nitrate solution --> Lead (IV) hydroxide + Magnesium nitrate. I know we have to know the polyatomic ions, but does the charge have anything to do with the numbers behind of the element? Why is hydroxide OH2 in the answer, when it's a OH- polyatomic ion? Does that have to do with balancing?

For cations that have ambiguous charge, the oxidation state is given by Roman numerals after the name. Something like sodium is pretty much always +1, but the transition metals and main group metals (like lead, tin, etc) can have a number of different stable charges, so these are specified. For this question, we need 2 hydroxides in the formula of magnesium hydroxide and 4 hydroxides in the formula of lead(IV) hydroxide. Hopefully it's "(OH)_2_" in the answer and not "OH2"...

Another question I have is number 11, winter exam 2006: How many grams of hydrogen are required to make 34.061g of ammonia by the following reaction? xH2(g)+ yN2(g) --> z NH3(g). I have no idea how to do this problem. Do you have to do something with mole ratio?

Yes, you need the mol ratio. First, balance the equation. Once you have correct numbers for x/y/z, then convert34.061g of ammonia into mols, use the mol ratio (x/z in this case) to convert mols of ammonia to mols of hydrogen, then use the molar mass of hydrogen to convert to grams.

How do you do concentration problems like numbers 12: 50.00mL of a 1.62 M potassium carbonate solution is diluted to 500.0mL. What is the concentration of potassium ions in the resulting solution, [K+]? and 13: what is the concentration of a perchloric acid stock solution if 21.53 mL of 1.054M Mg(OH)2(aq)is required to titrate 15.00mL of HClO4(aq) to the equivalence point in the following reaction?: a HClO4 (aq) + b Mg(OH)2 (aq) --> c H2O(aq) + Mg(ClO4)2(aq).on winter 2006 exam?

Hmm, this is a 2-fer. When you are diluting a solution of known concentration, use the formula C1V1 = C2V2 where C's are concentrations and V's are volumes. In this case, plugging in numbers gives:
(1.62M)(50.00mL) = C2(500.0mL)

The second one is a titration problem, which is just a specific type of stoichiometry problem. Write a balanced chemical equation, convert 21.53mL of 1.054M Mg(OH)2(aq) to mols, use the ratio from the balanced equation to convert mols Mg(OH)2 to mols HClO4, then use the given volume to convert mols HClO4 to concentration (mols/L).

Number 14, winter exam 2006: 50,00mL of 1.119M Co(NO3)3 (aq) is combined with 60.00mL 1.821 M Na2Co (aq). 2.946g of precipitate is recovered from this reaction. I understand parts a and b, but I don't understand c: What is the percent yield of this product?

Percent yield is the actual yield divided by the theoretical yield time 100%. Actual yield is the amount you collect or "recover" from the reaction, theoretical yield is the maximum possible amount you could produce if you use all of the limiting reagent to make product.

On exam fall 2006, number 9 is assigning oxidation numbers to each element: AgNO3. Ag is +1, N is +5 and O is -2. How are we suppose to know that? N's charge is -3, why is it's oxidation number +5? Also, how do you know the charge of transition elements?

If nitrogen were just some random nitrogen ion, we'd probably expect it to have a charge of -3, in that case it would be a "nitride". In this case, nitrogen is part of the polyatomic nitrate ion. The sum of all the oxidation numbers of the atoms in a polyatomic ion is equal to the charge of the polyatomic ion. For nitrate, we'd expect the oxygens to have oxidation numbers of -2, so:
(Ox# nitrogen) + 3(Ox# oxygen) = (charge of nitrate)
(Ox# nitrogen) + 3(-2) = (-1)
(Ox# nitrogen) = +5
For transition and main group metals (see one of the answers above), the Ox# will either be given as a Roman numeral, or it will have to be determined from a given formula. In this example, the charge of nitrate is -1, so if the given formula is "AgNO3", then the silver must have a charge of +1.

I hope this helps, I'll probably check in again a little later...