The oxidation state of Fe in FeSO4.(NH4)2SO4.6H2O is
Here, Sum of O. N. for H2O = 2(1) + (-2) = 0
Sum of O.N. for NH4 = -3 + 4(1) = +1
O.N. for SO4 = -2
Sum of O.N. for (NH4)2SO4 = (+1)2 + (-2) = 0
Therefore, FeSO4.(NH4)2SO4.6H2O ⇒ n + (-2) + 0 + 0 = 0
⇒ n = +2