$$\begin{gathered} {\mathrm{FeO}}_4^{2-}\stackrel{+2.2\mathrm{V}}{\to }{\mathrm{Fe}}^{3+}\stackrel{+0.70\mathrm{V}}{\to } \\ {\mathrm{Fe}}^{2+}\stackrel{-0.45\mathrm{V}}{\to }{\mathrm{Fe}}^0 \\ {{\mathrm{E}}^{\mathrm{o}}}_{{\mathrm{FeO}}_4^{2-}/{\mathrm{Fe}}^{2+}} \mathrm{is} \mathrm{x}\times {10}^{-3}\mathrm{V}.\mathrm{The}\mathrm{value}\mathrm{of} \mathrm{x} \mathrm{is} \end{gathered}$$

$\begin{array}{l}{\mathrm{FeO}}_4^{-2}\to {\mathrm{Fe}}^{+3},{\mathrm{\Delta G}}_0=-3\times \mathrm{F}\times 2.2\\ {\mathrm{Fe}}^{+3}\to {\mathrm{Fe}}^{+2},{\mathrm{\Delta G}}_0=-1\times \mathrm{F}\times 0.7\\ {\mathrm{FeO}}_4^{-2}\to {\mathrm{Fe}}^{+3},\mathrm{\Delta G}=-4\times \mathrm{F}\times {\mathrm{E}}_0\\ \therefore -4{\mathrm{FE}}_{\text{cell }}^0=-3\mathrm{F}\times 2.2-1\mathrm{F}\times 0.7,\\ {\mathrm{E}}_{\text{cell }}^0=\frac{3\times 2.2+0.7}{4}=1.825=1825\times {10}^{-3}\end{array}$