Q49 / 200 · NEET 2002 · Chemistry
Solution of $0.1 \mathrm{~N} \mathrm{NH}_4 \mathrm{OH}$ and $0.1 \mathrm{~N}$ $\mathrm{NH}_4 \mathrm{Cl}$ has $\mathrm{pH~} 9.25$, then find out $\mathrm{pK}_b$ of $\mathrm{NH}_4 \mathrm{OH}$.
- A9.25
- Correct answerB4.75
- C3.75
- D8.25
Solution
Concentration of salt
$\mathrm{NH}_4 \mathrm{Cl}=0.1 \mathrm{~N}$
Concentration of the base $\mathrm{NH}_4 \mathrm{OH}=0.1 \mathrm{~N}$
$\mathrm{pH}$ of solution is 9.25
$\therefore \mathrm{pOH}=14-\mathrm{pH}=14-9.25=4.75$
We know that,
$\mathrm{pOH}=\mathrm{pK}_{\mathrm{b}}+\log \frac{[\text { salt }]}{\text { [base }]}$
(But here concentration of salt $=$ concentrtion of base and $\left[\log _{10}(1)-\right.$ $0]$.
$\therefore \quad \mathrm{pOH}=p \mathrm{~K}_b=4.75$
A Caution
$\begin{aligned}
\mathrm{NH}_4 \mathrm{OH} & \rightleftharpoons \mathrm{NH}_4^{+} \mathrm{Cl}^{-} \\
\mathrm{NH}_4 \mathrm{Cl} & \rightarrow \mathrm{NH}_4^{+}+\mathrm{Cl}^{-}
\end{aligned}$
Due to the common ion effect, the reaction of dissociation of $\mathrm{NH}_4 \mathrm{OH}$ shifts to the left.
Hence, the correct answer is option: 'B'.