Hydrated ferrous ammonium sulphate is commonly known as mohr’s salt .It is prepared by crystallization of equimolar solution of FeSO4 and (NH4)2SO4.
FeSO4(aq) + (NH4)2SO4 crystallization → FeSO4 (NH4)2SO4.6H2O.
This double salt is more stable than FeSO4 alone and is used as reducing agent in redox fitration. During redox reaction Fe+2 ions mohr’s salt is oxidised to Fe+3 ions.
(Properties same as FeSO4. 7H2O)