The reaction given is Cr2O72−+Fe2++C2O42−⟶Cr3++Fe3++CO2 Cr2O72−⟶2Cr3+ On balancing 14H++Cr2O72−+6e−⟶2Cr3++7H2O . . . (i) Fe2+⟶Fe3++e− . . . (ii) C2O42−⟶2CO2+2e− . . . (iii) On adding all three equations, we get Cr2O72−+Fe2++C2O42−+14H++3e− →2Cr3++Fe3++2CO2+7H2O Hence the total no. of electrons involved in the reaction =3