The formula for ferrous ammonium sulphate is FeSO4.(NH4)2SO4.6H2O. It is prepared by dissolving equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water containing a little sulphuric acid. The solution is subjected to crystallisation, ferrous ammonium sulphate separates out from the solution.
The chemical reaction is given below.
FeSO4+(NH4)2SO4+6H2O→FeSO4.(NH4)2SO4.6H2O(Mohr’ssalt)
Avoid prolonged heating while preparing crystals of ferrous ammonium sulphate, as it may oxidise ferrous ions to ferric ions and change the stoichiometry of the crystals.