IIT JEE Advanced 2007 Paper 2

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.