The \(\mathrm{p} K_a\) of a weak acid, benzoic acid and \(\mathrm{p} K_b\) of a weak base, ammonium…
The \(\mathrm{p} K_a\) of a weak acid, benzoic acid and \(\mathrm{p} K_b\) of a weak base, ammonium hydroxide are 4.25 and 4.75 respectively. Then the \(\mathrm{pH}\) of \(0.1 \mathrm{M}\) solution of ammonium benzoate will be
7.10
7.50
6.75
6.50
Solution
Ammonium benzoate \(\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{COONH}_4ight)\) is the salt weak benzoic acid \(\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{COOH}, \mathrm{p} K_a=4.25ight)\) and weak ammonium hydroxide \(\left(\mathrm{NH}_4 \mathrm{OH}, \mathrm{p} K_b=4.75ight)\).
So, \(\mathrm{pH}\) of the solution of \(\mathrm{C}_6 \mathrm{H}_5 \mathrm{COONH}_4\) will be,
\(\begin{aligned}
\mathrm{pH} & =7+\frac{1}{2}\left(\mathrm{p} K_a-\mathrm{p} K_bight) \\
& =7+\frac{1}{2}(4.25-4.75)=7-0.25=6.75
\end{aligned}\)
.