Question

Why P.O.P is written as CaSO4.1/2H2O

Answer 1

it become gypsum if there are no water of crystallisation

Answer 2

CaSO4.1/2H2O is known as calcium sulphate hemihydrate.
In plaster of Paris 2 units of calcium sulphate share one molecule of water making it 1/2 molecule of water per CaSO4 unit.

Half a molecule of water means that two units of a chemical substance share one molecule of water.