The skeletal ionic equation is Cr2O7+Fe2+ → Fe3++Cr3+ First calculate the oxidation number of Cr 2x + 7(−2) = −2 2x −14 = −2⇒ x = +6 Oxidation No. increases by 1 unit (oxidation) Cr2O72−+Fe2+ → Fe3++Cr3+ In Cr2O7, the decrease in oxidation number is by 3 units per atom and since 2 atoms of chromium are involved so the total decrease is 3 × 2 = 6. Cr2O7+6e− → Cr3+ Fe2+ → Fe3++e− × 6 We need to multiply this equation by 6. So, the n factor for Cr2O7 = 6 and for Fe2+ = 1. So, to reduce 1 mol of dichromate, 6 moles of Fe2+ are required.