Question

Why is plaster of paris written as Caso4.1/2H2O? how is it possible to have half a water molecule attached to CuSO4?

Answer 1

HI there !

Plaster of Paris is calcium sulphate hemihydrate . The formula for Plaster of Paris is CaSO₄. $\frac{1}{2}$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.  $\frac{1}{2}$ H₂O+ 1 $\frac{1}{2}$ 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

Answer 2

Answer:

It is impossible to have 0.5 molecule of water. The formula CaSO₄.1/2 H₂O  means that two formula units of CaSO₄ shares one molecule of water  , in such way that the effective water of crystallisation of one unit of calcium sulphate comes to 1/2 molecule of water.

Explanation:

On heating gypsum at 373K, it loses water molecules and becomes calcium sulphate hemihydrate whixh is CaSO4.1/2H2O. This is known as Plaster of Paris.

It can also be denoted as 2CaSO4.H20

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