For a cell $2 \mathrm{M}_{(\mathrm{S})}+\mathrm{O}_2(\mathrm{~g})+4 \mathrm{H}^{+} \rightarrow 2 \mathrm{M}^{2+}(\mathrm{aq})+2 \mathrm{H}_2 \mathrm{O}_{(\mathrm{l})}$ the $\mathrm{E}^0$ cell $=+1.67 \mathrm{~V}$. When $\left[\mathrm{M}^{2+}\right]$ is $1.0 \times 10^{-3} \mathrm{M}$ and $\mathrm{p}\left(\mathrm{O}_2\right)$ is 0.1 atm , the EMF of the cell becomes +1.57 V . Calculate the pH of the electrochemical cell.

When $0.1 \mathrm{~mol} \mathrm{~L}^{-1}$ of KCl was filled in a Conductivity cell the resistance was 80 ohms at 298 K . (Conductivity of 0.1 M KCl at 298 K is $1.29 \mathrm{~S} \mathrm{~m}^{-1}$. The same cell when filled with an unknown electrolyte of concentration 0.025 M , had a resistance of 92 ohms. What is the Molar conductivity of the electrolyte at the given concentration?

Identify the two incorrect electrochemical reactions shown as taking place in the given cells at the respective electrodes.

Cell	Electrode	Reaction
Lead-Storage Cell	Cathode	$\mathrm{Pb}+2 \mathrm{SO}_4{ }^{2-}+4 \mathrm{H}^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{PbSO}_4+2 \mathrm{H}_2 \mathrm{O}+\mathrm{SO}_2$
Leclanche Cell	Cathode	$\mathrm{MnO}_2+\mathrm{NH}_4^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{MnO}+\mathrm{OH}+\mathrm{NH}_3$
Mercury Cell	Anode	$\mathrm{Zn} / \mathrm{Hg}+2 \mathrm{OH}^{-} \rightarrow \mathrm{ZnO}(\mathrm{s})+\mathrm{H}_2 \mathrm{O}+2 \mathrm{e}^{-}$
Fuel Cell	Cathode	$\mathrm{O}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}+4 \mathrm{e}^{-} \rightarrow 4 \mathrm{OH}^{-}$

The $E_{M^{3+} / M^{2+}}^o$ values for $\mathrm{Cr}, \mathrm{Mn}, \mathrm{Fe}$ and Co are $-0.41,+1.57,+0.77$ and +1.97 V respectively.

For which of these metals, the change in oxidation state from +2 to +3 is the easiest?