In an acidic buffer solution $(\mathrm{pH}=4.4)$, the concentration ratio of acid and salt is $2: 1$. The value of dissociation constant of weak acid may be

 The expression for the $\mathrm{pH}$ of the acidic buffer solution is as given below.

$\mathrm{pH}={\mathrm{pK}}_{\mathrm{a}}+\log \frac{\text{ [salt] }}{\text{ [acid] }}$

$\mathrm{pH}=4.4$

$\frac{\left[\mathrm{salt}\right]}{\left[\mathrm{acid}\right]}=\frac{1}{2}$

Substitute values in the above equation, we get:

$4.4={\mathrm{pK}}_{\mathrm{a}}+\log \left(\frac{1}{2}\right)$

Hence, ${\mathrm{pK}}_{\mathrm{a}}=4.7$

${\mathrm{pK}}_{\mathrm{a}}=-\log \left({\mathrm{K}}_{\mathrm{a}}\right)=4.7$

Hence, ${\mathrm{K}}_{\mathrm{a}}=2\times {10}^{-5}$

Therefore, the correct option is (D).