A buffer solution is prepared in which the concentration of $\mathrm{NH}_3$ is $0.30 \mathrm{M}$ and the…

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? $(\log 2.7=0.43)$
  1. 9.43
  2. 11.72
  3. 8.73
  4. 9.08

Solution

$\begin{aligned} \mathrm{pOH} & =\mathrm{p} K_{\mathrm{b}}+\log \frac{[\text { Salt }]}{[\text { Base }]} \\ & =-\log K_{\mathrm{b}}+\log \frac{[\text { Salt }]}{[\text { Base }]} \\ & =-\log 1.8 \times 10^{-5}+\log \frac{0.20}{0.30} \\ & =5-0.25+(-0.176) \\ & =4.75-0.176=4.57 \\ \therefore \mathrm{pH} & =14-4.57=9.43 \end{aligned}$

Asked in: NEET 2011 (Screening)

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