Question

The $p{K}_a$ of $HCOOH$ is $3.8$ and $p{K}_b$ of $N{H}_3$ is $4.8$, find the $pH$ of aqueous solution of $1MHCOON{H}_4$.

Answer 1

Answer: 6.5

$HCOON{H}_4$ is a salt of weak acid and weak base;
$pH=\frac{1}{2}p{K}_w+\frac{1}{2}p{K}_a-\frac{1}{2}p{K}_b$
$\therefore pH=\frac{1}{2}\times 14+\frac{1}{2}\times 3.8-\frac{1}{2}\times 4.8$;
$pH=6.5$