Question

In acid buffer solution (pH=4.4), the ratio of concentrations of acid to salt is 2 : 1. The value of dissociation constant of weak acid may be:

• A. $1.8\times {10}^{-4}$
• B. $2\times {10}^{-7}$
• C. $4\times {10}^{-5}$
• D. $2\times {10}^{-5}$

Answer 1

The correct option is D $2\times {10}^{-5}$

The expression for the pH of the acidic buffer solution is as given below.
$pH=p{K}_a+log\frac{\left[salt\right]}{\left[acid\right]}$
$pH=4.4$

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

Substitute values in the above equation.
$4.4=p{K}_a+log\left(\frac{1}{2}\right)$

Hence, $p{K}_a=4.7$

$p{K}_a=-log\left(K_a\right)=4.7$

Hence, $K_a=2\times {10}^{-5}$

Option D is correct.