Question

why plaster of Paris called as CaSo4.1/2H2O

Answer 1

due to the water of crystallisation.
plaster of Paris is known as caso4.1/2 h2o

Answer 2

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