Question

Which of the pairs below would be the best choice for a pH 5 buffer? HF/NaF, K a (HF) = 3.5 × 10-4 HC2H3O2/KC2H3O2, K a (HC2H3O2) = 1.8 × 10-5 NH3/NH4Cl, K b (NH3) = 1.8 × 10-5

Answer 1

Answer:

HC₂H₃O₂/KC₂H₃O₂

Explanation:

Considering the Henderson- Hasselbalch equation for the calculation of the pH of the basic buffer solution as:

$pH=pK_b+log\frac{[salt]}{[acid]}$

For a best pair, the pKa value must be equal to pH.

NH₃/NH₄Cl forms a basic buffer and cannot account for pH = 5

out of the acidic buffer given,

So, HF , Ka = 3.5 × 10⁻⁴ , So pKa = 3.46

HC₂H₃O₂ , Ka = 1.8 × 10⁻⁵ , So pKa = 4.77

The best pair to show pH = 5 is HC₂H₃O₂/KC₂H₃O₂