Question

A weak acid $HX$ has the dissociation constant $1\times {10}^{-5}M$ . It forms a salt $NaX$ on reaction with alkali. The percentage hydrolysis of $0.1M$ solution of $NaX$ is:

• A. $0.0001\%$
• B. $0.01\%$
• C. $0.1\%$
• D. $0.15\%$

Answer 1

The correct option is B $0.01\%$

$NaX+H_{2}O\rightleftharpoons NaOH+HX$
In this, $HX$ is a weak acid. So, $NaX$ is a salt of weak acid and strong base.
$\therefore$ Hydrolysis constant of $NaX$ is
$K_h=\frac{K_w}{K_a}=\frac{1\times {10}^{-14}}{{10}^{-5}}=1\times {10}^{-9}$
For this type of salt, $K_h=C{h}^2$
$C=\text{molar concentration}h=\text{degree of hydrolysis}$
$\therefore 1\times {10}^{-9}=0.1\times h^2$
$\Rightarrow h^2=\frac{1\times {10}^{-9}}{0.1}=1\times {10}^{-8}h=1\times {10}^{-4}$
$\therefore \text{percentage hydrolysis of}NaX\text{salt}=1\times {10}^{-4}\times 100=1\times {10}^{-2}=0.01\%$