Questions...

On exam #4 question #6: For each of the following reactions, predict whether the sign if delta S will be positive or negative and explain your answer. What are you looking for as an answer for the explanation part?

In the first one, I expected to see something like "A solid is forming from two solutions, so the products are more ordered/less disordered than the reactants." For the second, 2 molecules of gas are forming; for the third, the number of gas particles is changing. A number of people were giving explanations like "positive because the reaction is getting more disordered." That's not an explanation. If your answer to an "explain" question does not address the question "Why?", then it's probably not really explaining the answer.

In exam #3 question #1 could you explain again how to find H⁺ and OH^- as well as how to find K_a and K_b?

[H⁺] and [OH^-] are related by K_w. K_a and K_b are also related by K_w.
[H⁺] [OH^-] = K_w
(K_a)(K_b) = K_w
At 25degC, K_w = 10^-14

In exam #3 question #2: 1.28 mol K₂SO₃ + 1.24 mol HCl, can you explain why this does not result in an effective buffer?

This will not make an effective buffer because adding 1.24mols of HCl will almost bring you to the equivalence point in this titration, it will not give you a buffer. You could make a buffer by adding 0.64mol of HCl(aq) to 1.28mols of K₂SO₃(aq) because the resulting solution would contain 0.64mols of SO₃^2-(aq) {a weak base} and 0.64mols of HSO₃^-(aq) {its weak conjugate acid}. You could also make an effective buffer by adding 1.92mols of HCl to 1.28mols of K₂SO₃(aq) because the resulting solution would contain 0.64mols of HSO₃^-(aq) {a weak base} and 0.64mols of H₂SO₃(aq) {its weak conjugate acid}.

In exam #2 could you run through #12: Graphite (solid carbon) reacts with oxygen gas to form carbon monoxide gas. You have sealed 11.37g of graphite and 18.61g of oxygen in a 6.00L vessel and allowed the system to reach equilibrium at 73.91C. if the equilibrium constant value is 9.42x10^-6 at this temperature, what are the equilibrium concentrations of all reactants and products?

Start with a balanced chemical equation:
C(s) + O₂(g) <=> CO₂(g)

Since carbon is a solid in this reaction, it does not appear in the equilibrium constant expression, so we can put together a table {again, formatting tables in this program is rough, so I'll simply list the concentrations}:

[O₂]_initial = 18.61g / 31.998g/mol / 6.00L = 0.0969M
[CO₂]_initial = 0M

[O₂]_change = -xM
[CO₂]_change = +xM

[O₂]_equilibrium = (0.0969-x)M
[CO₂]_equilibrium = xM

Kc = [CO₂]_equilibrium / [O₂]_equilibrium = (x) / (0.0969-x) = 9.42x10^-6

This isn't a horrible equation to solve directly, but let's see if we can make an assumption. If "x" is much smaller than 0.0969M, then this expression simplifies to:

(x) / 0.0969 = 9.42x10^-6
x = 9.13x10^-7
{"x" is indeed much smaller than 0.0969, so our assumption is valid.}

[O₂]_equilibrium = 0.0969 M
[CO₂]_equilibrium = 9.13x10^-7 M