[Fluorine]

Questions:
1) What is the pH if "a" and "b" are mixed?
2) What is the pH i 3.6g of NaHSO₃ is added?

Info:
a) 0.096M Na₂SO₃, pH 3.91
b) 0.340M H₂SO₃, pH 1.07

Ka₁ = 1.7x10^-2 = [H⁺][HSO₃^1-]/[H₂SO₃]

Ka₂ = 6.4x10^-8 = [H⁺][SO₃^2-]/[HSO₃^1-]

K = 10^-14/6.4x10^-8 = 1.5625x10^-7

Attempt:
1) So for the first one I'm thinking I would use the Henderson-Hasselbach equation but I'm not sure which Ka to use for the pKa. So far I've got it up to here; where ? is pKa.

2) Not sure how to start this one.

[Borek]

SO₃^2- will react with H₂SO₃.

[Fluorine]

H₂SO₃ H⁺ + HSO₃^1- 2H⁺ + SO₃^2-

I was thinking along those lines, if I follow your hint correctly, but assumed I was wrong. Use pKa for whichever there is more of? In this case the sulfurous acid is in greater concentration than it's conjugate base.

If correct so far, I would use Ka₂ = 6.4x10^-8 as pKa = 3.2, OR 0.340 (acid) - 0.096 (c.base) = 0.244 [H⁺] ("excess") as pKa = 0.612. Not sure which it would be, the latter makes sense considering it's a buffer problem.

Is this on the right track or am I all off?

[Borek]

You are on the right track. Assume reaction went to completion - which acid and its conjugate base are present? System is dominated by the equilibrium between those two, that lets you select correct dissociation constant.

[Fluorine]

This is good to know, I did something very similar on a exam but could not finish due to time. Thankfully showing work counts quite a bit.

which acid and its conjugate base are present?

Sulfurous acid is in excess so equilibrium is favoring towards products;

 H₂SO₃ 2H⁺ + SO₃^2-

pH = 3.2 (pKa₂) + (-0.670941280736) = 2.5

Logically the answer makes sense to me, acid 'out-concnentrated' base and the pH is quite acidic. Though what confuses me is why only Ka₂ is used (assuming I'm correct) instead of both Ka₁ and Ka₂? Is it because HSO₃^1- is too minute for necessity of consideration?