The EMF of the cell $\mathrm{M}/{\mathrm{M}}^{{\mathrm{n}}^{+}}(0.02\mathrm{M})/{\mathrm{IH}}^{+}\left(1\mathrm{M}\right)\mid {\mathrm{H}}_2( \mathrm{g})(1 \mathrm{atm}),\mathrm{Pt}$ at $25°\mathrm{C}$ is $0.81 \mathrm{V}.$ Calculate the valency of the metal if the standard oxidation potential of the metal is $0.76 \mathrm{V}.$

During the discharge of a lead storage battery, the density of sulphuric acid fell from $1.294$ to $1.139\mathrm{g}$ ${\mathrm{mL}}^{-1}·{\mathrm{H}}_2{\mathrm{SO}}_4$ of density $1.294{\mathrm{gmL}}^{-1}$ is $39\%$ and that of density $1.139 \mathrm{g}·{\mathrm{mL}}^{-1}$ is $20\%$ by weight. The battery holds $3.5 \mathrm{L}$ of acid and the volume practically remains constant during discharge. Calculate the number of ampere hours for which the battery must have been used. The discharging and charging reactions are.

$\mathrm{Pb}+{\mathrm{SO}}_4^{2-}\to {\mathrm{PbSO}}_4+2e^{-}$                                         (Charging)
${\mathrm{PbO}}_2+4{\mathrm{H}}^{+}+{\mathrm{SO}}_4^{2-}+2e^{-}\to {\mathrm{PbSO}}_4+2{\mathrm{H}}_2\mathrm{O}$        (discharging)