A litre of buffer solution contains 0.1 mole of each of ${\mathrm{NH}}_3$  and ${\mathrm{NH}}_4\mathrm{C}\mathcal{l}$ . On the addition of  0.02 mole of $\mathrm{HC}\mathcal{l}$  by dissolving gaseous $\mathrm{HC}\mathcal{l}$ , the pH of the solution is found to be ______×10^-3  (Nearest integer)
[ Given :-  ${\mathrm{pK}}_{\mathrm{b}}\left({\mathrm{NH}}_3\right)=4.745$
$\log 2=0.301$
$\log 3=0.477$
$\mathrm{T}=298\hspace{0.17em}\hspace{0.17em}\mathrm{K}$]