Question

What will be the $pH$ of $0.1M$ ${CH}_{3}COO{NH}_4$? Dissociation constants of ${CH}_{3}COOH$ and ${NH}_{4}OH$ are $K_a=1.8\times {10}^{-5}$ and $K_b=1.8\times {10}^{-5}$ respectively.

Answer 1

The salt ${CH}_3{COONH}_4$ is formed from weak acid ${CH}_{3}COOH$ and weak acid base ${NH}_{4}OH$.

${CH}_3{COONH}_4+H_{2}O\rightleftharpoons {CH}_{3}COOH+{NH}_{4}OH$

$pH=\frac{1}{2}{pK}_w-\frac{1}{2}{pK}_a-\frac{1}{2}{pK}_b$

$=\frac{1}{2}\times 14+\frac{1}{2}\times 4[-log18\times {10}^{-5}]-\frac{1}{2}[-log\times 1.8\times {10}^{-5}]$

$=7$