Chemistry - Ionic Equilibrium Question with Solution | TestHub

$\underset{(\text{ weak base })}{{\mathrm{CH}}_3{\mathrm{NH}}_2}+$ & $\stackrel{\mathrm{HCl}}{(\text{ strong acid })}\rightleftharpoons$ & $\underset{(\text{ salt })}{{\mathrm{CH}}_3\stackrel{+}{\mathrm{N}}}{\mathrm{H}}_3{\mathrm{Cl}}^{-}$

Initially 0.2 mole 0.18 mole 0

After $0.20-0.18$ $(0.18-0.18)$ 0.18 mole

$=0.02$ mole $=0$

It is clear from above calculation, weak base and its salt both are present in mixture.

Hence it work as basic buffer.

For basic buffer $\left[{\mathrm{OH}}^{-}\right]={\mathrm{K}}_{\mathrm{b}}\times \frac{[\text{ Base }]}{[\text{ Salt }]}$

$$\begin{array}{rlrl}& \left[{\mathrm{OH}}^{-}\right]=\frac{\left(1.8\times 10^{-4}\right)\times 0.02}{0.18}& =\frac{1.8\times 10^{-4}\times 2}{18}& =2\times 10^{-5}\mathrm{M}\end{array}$$