Question

Why is Plaster of Paris written as CaSO4.1/2H2O?
How is it possible to have half a molecule of water attached to it?

Answer 1

It means that one molecule of Calcium sulphate contains 1/2 molecule of H2O .

So we can write that 2 molecules of calcium sulphate contains 1 molecule of H2O .

So the formula can also be written as

(CaSO4)2•H2O

Answer 2

Explanation:

Plaster of Paris is calcium sulphate hemihydrate . The formula for Plaster of Paris is CaSO₄. H₂O.

Plaster of Paris is prepared from Gypsum. Gypsum is  calcium sulphate dihydrate [ CaSO₄. 2 H₂O ] 

Plaster of Paris is prepared by heating Gypsum to a temperature of 100°C in a kiln.

When gypsum is heated , it loses three fourth of its water of crystallization and forms plaster of Paris .

CaSO₄. 2 H₂O     ----------------> CaSO₄.   H₂O+ 1  H₂O

It is not possible to have half molecule of water. The formula CaSO₄.1/2 H₂O  means that two  formula units of CaSO₄ shares one molecule of water  , in such a way  that the effective water of crystallisation of one  particular unit of calcium sulphate comes to 1/2 molecule of water.

Plaster of Paris  =  2CaSO₄.H₂O  , OR , CaSO₄.1/2 H₂O

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