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

Please explain to me how to solve this, and also tip to see if a compound is weak/strong and acid/base.

Answer 1

No. 1 is a buffer because it has a weak acid (CH3COOH) and a conjugate base (CH3COO^-). The CH3COO^- arises from the addition of the KOH. CH3COOH + OH^- ==> CH3COO- + H2O

No.2 is a buffer because is has a weak acid (H₂SO₃) and a conjugate base (HSO₃^-). The conjugate base arises from the addition of NaOH. H₂SO₃ + OH^-==> HSO₃^- + H2O ==> SO₃^2- + H2O. The generation of HSO₃^- or SO₃^2- depends on the moles of OH^- added. Here the first 0.05 mols OH^- added produces HSO₃^- and then after that, additional OH^- produces SO₃^2-.

No. 3 is a buffer because it has a weak acid (H₂SO₃) and a conjugate base SO₃^2-.

So, answer should be D. All three mixtures are buffers.

Tip:

If the compound has a Ka or Kb, it is a WEAK acid/base respectively.