Question

The $p{K}_a$ of $HCN$ is $9.3$. The $pH$ of a solution prepared by mixing $2.5$ moles of $HCN$ and $0.25$ moles of $KCN$ in water and making of the total volume to $500$ mL is:

• A. $9.3$
• B. $7.3$
• C. $10.3$
• D. $8.3$

Answer 1

The correct option is D $8.3$

A mixture of $HCN$ and $KCN$ is an acidic buffer.

The expression for the $pH$ of the acidic buffer solution is as given below:

$pH=p{K}_a+log\frac{\left[salt\right]}{\left[acid\right]}$

$p{K}_a=9.3$

Substituting values in the above equation, we get

$pH=9.3+log\frac{0.25}{2.5}=8.3$.

Option D is correct.