Question

What do you mean by it is written in this form because 2 formula units of CaSo4 share 1 molecule of water???

Answer 1

The 2 water molecules joint to the CaSO4 molecule represent the water of crystallisation, water of crystallisation is one of the characteristic property of crystalline solids (class 11 topic)

When hydrated salts are heated strongly,they lose their water of crystallisation.By losing water of crystallisation,the hydrated salts lose their regular shape and colour,and become colourless powdery substances,The salts which have lost their water of crystallisation are called anhydrous salts.

For Ex:Copper sulphate crystals are blue in colour.When copper sulphate crystals are heated strongly,they lose water of crystallisation and form anhydrous copper sulphate which is white in colour.

Answer 2

Plaster of paris is Calcium sulphate hemihydrate . [CaSO4.1/2 H2O]
Please Note that it is not possible to have a half a molecule of water.
The Formula actual means that two molecules ( or two formula units ) of CaSO4 share one moecule of water so that the effective water of crystallisation for one CaSO4 unit comes to half molecule of water.
The formula of plaster of paris can be written as 2CaSO4.H2O
2(CaSO4.2H2O)  -----> 2CaSO4.H20  + 3 H2O
                           on heating