Q52 / 160 · JIPMER 2014 · Chemistry

A buffer solution is prepared in which the concentration of $\mathrm{NH}_3$ is 0.30 M and the concentration of $\mathrm{NH}_4^{+}$is 0.20 M . If the equilibrium constant, $K_b$ for $\mathrm{NH}_3$ equals $1.8 \times 10^{-5}$, what is the pH of this solution? $(\log 2.7=0.43)$

  • Correct answer
    A
    $9.43$
  • B
    $11.72$
  • C
    $8.73$
  • D
    $9.08$

Solution

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

Hence, the correct answer is option: 'A'.

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