Question

At ${25}^{\circ }C$ disociation constant of acid $HA$ and base $BOH$ in aqueous solution are same. The $pH$ of $0.01M$ solution of $HA$ is $5$. The $pOH$ of ${10}^{-4}$ M solution of $BOH$ at the same temperature is:

• A. $3.5$
• B. $4$
• C. $6$
• D. None of these

Answer 1

The correct option is C $6$

For $0.01M$ solution of $HA$
$pH=\frac{1}{2}p{K}_a-\frac{1}{2}\log C\Rightarrow 5=\frac{1}{2}p{K}_a+\frac{1}{2}\times 2p{K}_a=8\Rightarrow p{K}_b=8;\text{For}{10}^{-4}\text{M solution of BOH}pOH=\frac{1}{2}p{K}_b-\frac{1}{2}\log C=4+2=6$