Stannous sulphate `(SnSO_(4))` and potassium permanganate are used as oxidising agents in acidic medium for oxidation of ferrrous ammnium sulphate to ferric sulphate. The ration of number of moles of stannous sulphate required per mole of ferrous ammonium sulphate to the number of moles of `KMnO_(4)` required per mole of ferrous ammonium sulphate, is: