Question

The pH of $0.1M$ solution of the following salts increases in the order :

• A. $NaCl<N{H}_{4}Cl<NaCN<HCl$
• B. $HCl<N{H}_{4}Cl<NaCl<NaCN$
• C. $NaCN<N{H}_{4}Cl<NaCl<HCl$
• D. $HCl<NaCl<NaCN<N{H}_{4}Cl$

Answer 1

Answer: (B)

(i) $HCl⟶{\underset{0.1}{H}}^{+}+\underset{0.1M}{C{l}^{-}}$
$\therefore \left[H^{+}\right]=0\cdot 1M$
$pH=-\log \left[H^{+}\right]=-\log 0\cdot 1=1$
(ii) $NaCl$ is a salt of strong acid and strong base so it is not hydrolysed and hence its $pH$ is $7.$
(iii) $N{H}_{4}Cl+H_{2}O\rightleftharpoons N{H}_{4}OH+HCl$
$\therefore$ The solution is acidic and its $pH$ is less than that of $0.1MHCl$.
(iv) $NaCN+H_{2}O\rightleftharpoons NaOH+HCN$
$\therefore$ The solution is basic and its $pH$ is more than that of $0.1MHCl$.
$\therefore$ Correct order for increase in $pH$ is
$HCl<N{H}_{4}Cl<NaCl<NaCN$.