Question

The $p K_{a}$ of a weak acid, benzoic acid and $p K_{b}$ of a weak base, ammonium hydroxide are $4.25$ and $4.75$ respectively. Then, the $pH$ of $0.1\, M$ solution of ammonium benzoate will be__________

• A. 7.10
• B. 7.50
• C. 6.75
• D. 6.50

Answer 1

Answer: (C)

Ammonium benzoate $\left( C _{6} H _{5} COONH _{4}\right)$ is the salt weak benzoic acid $\left( C _{6} H _{5} COOH , p K _{a}=4.25\right)$ and weak ammonium hydroxide $\left( NH _{4} OH , p K_{b}=4.75\right)$
$p H=7+\frac{1}{2}\left(p K_{a}-p K_{b}\right)$
$=7+\frac{1}{2}(4.25-4.75)=7-0.25=6.75$