Ammonium iron (II) sulphate crystals have the following formula: (NH₄)₂SO₄·FeSO₄·nH₂O. In an experiment to find n, 8.492 g of the salt were dissolved and made up to 250 cm³ solution with distilled water and dilute sulphuric acid. A 25.0 cm³ portion of the solution was titrated against 0.0150 mol dm⁻³ KMnO₄, 22.5 cm³ being required. Calculate n.