Given standard electrode potentials
Fe+++2e−→Fe; E∘=−0.440 V
Fe++++3e−→Fe; E∘=−0.036 V
The standard electrode potential (E∘) for Fe++++e−→Fe++ is
+ 0.772 V
ΔG∘=−nE∘F
Fe2++2e−→Fe .......(i)
ΔG∘=−2×F×(−0.440 V)=0.880 F
Fe3++3e−→Fe ......(ii)
ΔG∘=−3×F×(−0.036)=0.108 F
On substracting equation (i) from (ii)
Fe3++e−→Fe2+
ΔG∘=0.108 F−0.880 F= −0.772F
E∘ for the reaction =−ΔG∘nF=−(−0.772 F)1×F=+0.772V