[Rachel492480]

So, my professor keeps telling me that my answers are wrong and I don't know WHY they are wrong. He just keeps hinting and asking me more questions hoping I will understand what I did wrong. Which I've repeatedly said that asking me more questions isn't going to give me an epiphany and that it's just annoying.
So, I'm putting my questions and the answers I wrote and I hope someone can help me figure out what I did wrong and actually TELL me what I did wrong so I can finish this class.

6. What is the [H+] in a 4M solution of butanoic acid? Butanoic acid partially dissociates forming an equilibrium:
H(C4H7O2)→ ←  C4H7O2-+ H+,
where the Ka of butanoic acid is 1.48 x 10-5.

1.48 * 10-5 = [H+][OH-] / 4M

1.48 *10 -5 = [X][X] / 4M

1.48*10 -5 (4M) = X2

x = 7.7 * 10-3 = H+

7. What is the pH of a 2M solution of uric acid?
HC5H3N4O3 → ← H+ + C5H3N4O3-
with Ka = 1.29 x 10-4.

1.29 * 10-4 = [H+][OH-] / [2M]
(1.29 * 10-4)(2M) = X2
2.58*10-4 = X2
sqrt(2.58*10-4) = X = .01606M
(.01606M)(.01606M) / 2 = 1.29*10-4M
[H+] = 1.606*10-2M
pH = -log(1.606*10-2) = 1.79
pH = 1.79

8. What is the pH of a 0.6M solution of NH3?

NH3 + H2O → ← NH4+ + OH-
1.8 x 10-5M = [OH-][H+] / [.6M]
(1.8*10-5M)(.6M) = X2
1.08*10-5 M= X2
sqrt(1.08*10-5M) = 3.29*10-3M = X
H+ = 3.29 *10-3 M
[3.29 *10-3M]2 / .6M = 1.8 *10-5M
pH = -log(3.29 * 10-3M) = 2.48

17. A solution of formic acid is found to have a pH of 2.0. What is the molarity of the solution of formic acid (Ka = 1.78 x 10-4)
H(CHO2) → ← H+ + CHO2-
H+ = 10-pH = .01
H+ = OH- = H(CHO2)

1.78 * 10-4M = (.01M)(.01M) / (.01M)
1.78 * 10 -4M = (1*10-4 M)/ (H(CHO2)
H(CHO2) = (1*10-4 M) / (1.78*10-4 M) = .562M

[rabolisk]

For 6 and 7, the answers are correct, although you should realize, especially for 7, the difference between formal or initial concentration and the equilibrium concentration. For 7, the initial concentration of uric acid was 2M, but the equilibrium concentration is not. Hence x²/ 2 = 1.29 * 10^-4 is not correct. Nevertheless, your answer and the actual answer is very close to each other, which is why I consider it correct. More importantly... [H⁺][OH^-] / [HA] = Ka is wrong. You're implying in both 6 and 7 that [H⁺] = [OH^-], which is clearly wrong. Review your book on weak acid dissociation. Your professor probably looked at this and counted it wrong.

8 has ammonia, which is a base. Again, [H⁺] does not equal [OH^-]. Review what Ka and Kb mean for weak acids and weak bases.

17 Again, [H⁺] does not equal [OH^-] which does not equal [HCOOH]. Don't confuse stoichiometry with equilibrium concentrations. If you have a reaction of the form

A  B + C

where A, B, and C are the substrates, and the coefficients are all 1, that does NOT mean that [A] = = [C]. What it means is that for one molecule of A that undergoes this reaction, one molecule of B and one molecule of C are formed.

[Rachel492480]

I don't have a book. It's an online lesson, and it doesn't really explain it well. If I can't assume that H+ = OH-, then how do I find out what each is if the question didn't tell me one of them already? 

[AWK]

7. What is the pH of a 2M solution of uric acid?
HC5H3N4O3 → ← H+ + C5H3N4O3-
with Ka = 1.29 x 10-4.

You can never obtain such a concentration of uric acid. 1 g of uric acid is soluble in 15 L of water.

[rabolisk]

I don't have a book. It's an online lesson, and it doesn't really explain it well. If I can't assume that H+ = OH-, then how do I find out what each is if the question didn't tell me one of them already? 

Here are some links to help you.

http://www.files.chem.vt.edu/RVGS/ACT/notes/Acid-Base_Equilibrium.html
http://www.chemguide.co.uk/physical/acideqiamenu.html

I can clear up any confusions you might still have afterwards.