To Prepare a Pure Sample of Ferrous Ammonium Sulphate

 Aim- To prepare double salts: ferrous ammonium sulphate (Mohr’s salt) 

Theory

When a mixture containing equimolar proportions of ferrous sulphate and ammonium sulphate is crystallised from its solution, a double salt is formed. The formation of double salt may be shown as follows:

(FeSO4+ (NH4)2SO4+ 6H2O →FeSO4. (NH4)2SO4. 6H2O  

       Ferrous ammonium sulphate (Mohr’s salt)

Fe2+ ions undergo hydrolysis, therefore, while preparing aqueous solutions of ferrous sulphate and ammonium sulphate in water, 2-3 mL dilute

sulphuric acid is added to prevent the hydrolysis of these salts

Requirements

Two beakers (250 ml), china-dish, funnel, funnel-stand, glass-rod, wash-bottle, tripod stand and wire-gauze. Ferrous sulphate crystals, ammonium sulphate crystals, dilute sulphuric acid and ethyl alcohol.

Procedure

1.Take a 250 ml beaker and wash it with water. Transfer 7.0 g ferrous sulphate and 3.5 g ammonium sulphate crystals to it. Add about 2-3 ml of dilute sulphuric acid to prevent the hydrolysis of ferrous sulphate.

2.In another beaker boil about 20 ml of water for about 5 minutes to expel dissolved air.

3. Add the boiling hot water to the contents in the first beaker in small instalments at a time. Stir with a glass rod until the salts have completely dissolved.

4. Filter the solution to remove undissolved impurities and transfer the filtrate to a china-dish.

5. Heat the solution in the china-dish for some time to concentrate it to the crystallisation point.

6. Place the china-dish containing saturated solution over a beaker full of cold water. On cooling crystals of Mohr’s salt separate out.

7. Decant off the mother liquor quickly. Wash the crystals in the china-dish with a small quantity of alcohol to remove any sulphuric acid sticking to the crystals.

8. Dry the crystals by placing them between filter paper pads.

Observations
Weight of crystals obtained =……….. g
Expected yield = ………..g
Colour of the crystals = …………..
Shape of the crystals =…………
The crystals of Mohr’s salt are monoclinic in shape.

Precautions

1. Cool the solution slowly to get good crystals.
2. Do not disturb the solution while it is being cooled.
3. Do not heat the solution for a long time as it may oxidize ferrous ions to ferric ions.