Question

40 ml, 0.1 M ammonia solution is mixed with 20 ml 0.1 M HCl, what is pH of mixture given pKb of NH3 solution= 4.74? Answer is 9.26

Answer 1

According to the Henderson-Hasselbach equation, pH of such buffer solution can be calculated as:
$$\begin{gathered} \mathrm{pOH}\hspace{0.17em}=\mathrm{pKb}+\log \frac{\left[\mathrm{Salt}\right]}{\left[\mathrm{Base}\right]} \\ \mathrm{At}\mathrm{equilibrium},\left[\mathrm{Salt}\right]=\left[\mathrm{Base}\right] \\ \mathrm{pOH}\hspace{0.17em}=\mathrm{pKb}=4.74 \\ \mathrm{pH}=14-\mathrm{pOH} \\ =14-4.74=9.26 \end{gathered}$$