$M + M ^{2+} \rightarrow M ^{2+}+ M $

$10^{-4} \quad 10^{-2}$

The Nernst equation for the above cell is calculated as follows.

$E _{\text {cell }}= E _{\text {cell }}^{\circ}-\frac{-2.303 RT }{ nF } \log \frac{[ P ]}{[ R ]}$

$=4-\frac{-2.303 RT }{2 F } \log \frac{10^{-2}}{10^{-4}}$

$=4-\frac{ RT }{ F } \log 10 \quad\left(\frac{ RT }{ F } \log 10=0.06\right)$

$=3.94$