buffer solution - chemistry

Question

I have no idea where to start on this. Please give me a well-detailed explanation of how to solve this.

Given:

pKa; (CH3COOH) 4.75

pKa1 (H2SO3) = 1.90

pKa2 (H2SO3) = 7.20

Which of the following mixtures is (are) not a buffer mixture(s)?

1. A mixture of 0.1 M CH 3 COOH and 0.05 M KOH.

2. A mixture of 0.05 M H2SO3 and 0.075 M NaOH.

3. A mixture of 0.15 M H2SO3 and 0.15 M K2SO3.

A. Mixture 1 and 2.

B. Mixture 3.

C. Mixture 2

D. All three mixtures are buffer mixture

(answer is B)

Answer 1

A buffer must contain a weak acid plus the conjugate base, OR a weak base plus the conjugate acid.

1) is a buffer since it contains a weak acid (CH3COOH) and the conjugate base (CH3COO^-). The conjugate base arises when you add KOH

2) is a buffer because you have a weak acid (H₂SO₃) and the conjugate base (HSO₃^-). The conjugate base arises when you ad NaOH

3) is NOT a buffer because although you have a weak acid (H₂SO₃), you do NOT have the appropriate conjugate base (HSO₃^-). Instead, you have SO₃^2-.