3.92 g of ferrous ammonium sulphate are dissolved in 100 ml water. 20ml of this solution requires 18 ml of potassium permaganate during titration for complete oxidation. The weight of KMnO₄ present in one litre of the solution is

Mohr's salt is FeSO₄.(NH₄)₂SO₄.6H₂O
GMW of Mohr's salt = 392g
n-factor Mohr's salt = 1(Fe⁺² →Fe⁺³)
GMW of KMnO₄ = 158g
n-factor KMnO₄/H⁺ = 5(MnO₄^- →Mn⁺² )
I W_{Mohr's salt} = 3.92g
V_sol = 100ml
 

20ml of this solution required 18ml of KMnO₄/H⁺ for complete oxidation
Gram equivalents of Mohr's Salt = Gram equivalents of KMnO₄ H⁺
V_aN_a = V_bN_b
20(0.1)1 = 18(M_b)5
 

                                                                                     = 3.511g