Chemistry - Electrochemistry Question with Solution | TestHub

Consider the following electrochemical cell at standard condition.
$\begin{array}{rl}& \mathrm{Au}(\mathrm{s})\left|{\mathrm{QH}}_2,\mathrm{Q}\right|{\mathrm{NH}}_4\mathrm{X}(0.01\mathrm{M})||{\mathrm{Ag}}^{+}(1\mathrm{M})\mid \mathrm{Ag}(\mathrm{s})\\ & {\mathrm{E}}_{\text{cell }}=+0.4\mathrm{V}\end{array}$
The couple ${\mathrm{QH}}_2/\mathrm{Q}$ represents quinhydrone electrode, the half cell reaction is given below

The ${\mathrm{pK}}_{\mathrm{b}}$ value of the ammonium halide salt $\left({\mathrm{NH}}_4\mathrm{X}\right)$ used here is _________. (nearest integer)