The dissociation constant of a substituted benzoic acid at $25^{\circ} \mathrm{C}$ is $1.0 \times 10^{-4}$.…
The dissociation constant of a substituted benzoic acid at $25^{\circ} \mathrm{C}$ is $1.0 \times 10^{-4}$. The $\mathrm{pH}$ of $0.01 \mathrm{M}$ solution of its sodium salt is
Solution
$
\text { The hydrolysis reaction of conjugate base of acid is }
$
$
\begin{gathered}
A^{-}(a q)+\mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{HO}^{-}+\mathrm{HA} \\
K_h=\frac{K_w}{K_a}=\frac{10^{-14}}{10^{-4}}=10^{-10}
\end{gathered}
$
Since, degree of hydrolysis is negligible;
$
\left[\mathrm{OH}^{-}\right]=\sqrt{K_h C}=10^{-6} \cdot p[\mathrm{OH}]=6 \text { and } \mathrm{pH}=14-6=8
$