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Preparation of Double Salts (Class 12)

1. Preparation of Double Salt - Preparation of Mohr’s Salt


Mohr’s Salt is Ferrous Ammonium Sulphate. FeSO4(NH4)2SO4.6H2O. It is a double salt of ferrous sulphate and ammonium sulphate. It is prepared by mixing solutions of ferrous sulphate and ammonium sulphate containing equimolar proportions of the two substances.


Procedure



A solution containing 24g of FeSO4.7H2O in 100mL of water is mixed with a solution of 11.4g of (NH4)2SO4 in 100mL of water. The mixture is heated until its volume is reduced almost to half and then it is filtered hot. The clear solution is set aside in an evaporating dish. Light green crystals of ferrous ammonium sulphate [FeSO4(NH4)2SO4.6H2O] will be deposited. It is filtered at the pump. The crystals are washed with a little distilled water and then dried in a current of air.


Ferrous ammonium sulphate is a light green crystalline substance. It is stable to oxidation and is not dehydrated on standing. So its is preferred to FeSO4.7H2O which is efflorescent and is liable to oxidation by atmospheric air.


2. Preparation of Double Salt - Preparation of Potash Alum


Potash alum is potassium aluminium sulphate, or K2SO4 Al2(SO4)3 24H2O. It is a double salt and is usually called alum and is prepared by mixing solutions containing equimolar proportions of K2SO4 and Al2(SO4)3


Procedure


6g of K2SO4 in 100 mL of water is mixed with 23g of Al2(SO4)3. 18H2O in 100mL of water. The solution is heated, evaporated to almost half the volume and then filtered hot. The clear solution is kept in an evaporating dish. Crystals of the alum will be deposited. It is filtered and then washed at the pump with a little distilled water and then it is dried in a current of air. It is a colourless octahedral shaped substance. It is used for the purification of water and as a mordant.

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