A buffer solution is a mixture that resists changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base (or a weak base and its conjugate acid).

Let's analyze each pair:

1. HNO₃ and CH₃COOH: HNO₃ is a strong acid, CH₃COOH is a weak acid. Mixing two acids (one strong, one weak) does not form a buffer. The strong acid will dominate the pH.

2. HNO₃ and CH₃COONa: HNO₃ is a strong acid, CH₃COONa is the salt of a weak acid (CH₃COOH). When mixed, they react: $\mathrm{HNO}{3}_{}+\mathrm{CH}{3}_{}\mathrm{COONa}\to \mathrm{CH}{3}_{}\mathrm{COOH}+\mathrm{NaNO}{3}_{}$ This produces the weak acid CH₃COOH and the salt NaNO₃ (from strong acid and strong base, neutral). The resulting solution contains CH₃COOH (weak acid) and its conjugate base (from any excess CH₃COONa if not fully reacted). If mixed in appropriate ratios (e.g., not complete neutralization), it can form a buffer.

3. KOH and CH₃COONa: KOH is a strong base, CH₃COONa is the salt of a weak acid. Mixing a strong base with a salt does not directly form a buffer; no weak acid is present initially. They do not react to produce a conjugate pair.

4. CH₃COOH and CH₃COONa: This is a classic buffer pair. CH₃COOH is the weak acid, and CH₃COONa provides the conjugate base (CH₃COO^-).

Conclusion: The pairs that can form a buffer are HNO₃ and CH₃COONa (if mixed such that some CH₃COONa remains after partial reaction) and CH₃COOH and CH₃COONa.

Related Topics and Formulae

Buffer Action: Buffers work based on the equilibrium between a weak acid (HA) and its conjugate base (A^-). The pH is given by the Henderson-Hasselbalch equation:

For a buffer to be effective, the ratio [A^-]/[HA] should be between 0.1 and 10.

Formation by Partial Neutralization: A buffer can also be prepared by partially neutralizing a weak acid with a strong base, or a weak base with a strong acid. In the case of HNO₃ and CH₃COONa, it is analogous to adding strong acid to the salt of a weak acid.