The efficiency of a fuel cell is given by

The equilibrium constant of the reaction:
Cu_(s) + 2Ag⁺_(aq) $$ \to $$ Cu²⁺_(aq) + 2Ag_(s);
E^o = 0.46 V at 298 K is

E^o_Fe²⁺/Fe = $$-$$ 0.441 V and E^o_{Fe³⁺/Fe²⁺} = 0.771 V, the standard EMF of the reaction Fe + 2Fe³⁺ $$ \to $$ 3Fe²⁺ will be

A hypothetical electrochemical cell is shown below.

$$A\left| {{A^ + }\left( {xM} \right)} \right|\left| {{B^ + }\left( {yM} \right)} \right|B$$

The emf measured is + 0.20 V. The cell reaction is