Which of the following salts turns red litmus blue in its aqueous solution?
Which of the following salts turns red litmus blue in its aqueous solution?
$\mathrm{NH}_4 \mathrm{NO}_3$
$\mathrm{NH}_4 \mathrm{Cl}$
$\mathrm{NH}_4 \mathrm{CN}$
$\mathrm{NH}_4 \mathrm{~F}$
Solution
$\mathrm{NH}_4 \mathrm{CN}$ is the salt of weak acid $\mathrm{HCN}$ $\left(\mathrm{K}_{\mathrm{a}}=4.0 \times 10^{-10}\right)$ and weak base $\mathrm{NH}_4 \mathrm{OH}$ $\left(\mathrm{K}_{\mathrm{b}}=1.8 \times 10^{-5}\right)$ showing that $\mathrm{K}_{\mathrm{a}} < \mathrm{K}_{\mathrm{b}}$. If $\mathrm{K}_{\mathrm{a}} < \mathrm{K}_{\mathrm{b}}$, the aqueous solution of the salt will be basic. Thus, the solution of $\mathrm{NH}_4 \mathrm{CN}$ is basic and turns red litmus blue.