Q76 / 200 · NEET 2011 (Screening) · Chemistry
A buffer solution is prepared in which the concentration of $\mathrm{NH}_3$ is $0.30 \mathrm{M}$ and the concentration of $\mathrm{NH}_4^{+}$is $0.20 \mathrm{M}$. If the equilibrium constant, $K_b$ for $\mathrm{NH}_3$ equals $1.8 \times 10^{-5}$, what is the $\mathrm{pH}$ of this solution?
$(\log 2.7=0.43)$
- Correct answerA9.43
- B11.72
- C8.73
- D9.08
Solution
$$
\begin{aligned}
\mathrm{pOH} & =\mathrm{p} K_{\mathrm{b}}+\log \frac{[\text { Salt }]}{[\text { Base }]} \\
& =-\log K_{\mathrm{b}}+\log \frac{[\text { Salt }]}{[\text { Base }]} \\
& =-\log 1.8 \times 10^{-5}+\log \frac{0.20}{0.30} \\
& =5-0.25+(-0.176) \\
& =4.75-0.176=4.57 \\
\therefore \mathrm{pH} & =14-4.57=9.43
\end{aligned}
$$
Hence, the correct answer is option: 'A'.