A litre of buffer solution contains 0.1 mole of each of NH_3 and NH_4 Cl. On the addition of 0.02 mole of HCl by dissolving gaseous HCl, the pH of…

A litre of buffer solution contains $\displaystyle 0.1$ mole of each of $\displaystyle \mathrm{NH}_3$ and $\displaystyle \mathrm{NH}_4 \mathrm{Cl}$. On the addition of $\displaystyle 0.02$ mole of HCl by dissolving gaseous HCl , the pH of the solution is found to be $\displaystyle \_\_\_\_$ $\displaystyle \times 10^{-3}$ (Nearest integer) [Given : $\displaystyle \mathrm{pK}_{\mathrm{b}}\left(\mathrm{NH}_3\right)=4.745$ $$\begin{aligned} & \log 2=0.301 \\ & \log 3=0.477 \\ & T=298 \mathrm{~K}] \end{aligned} $$