Water of crystallisation is the fixed number of water molecule present in the one formula unit of a salt. One formula unit of copper sulphate contains five water molecule.
When the hydrated salt is heated they lose the water molecule. From the difference in weight of the hydrated and anhydrated the number of water molecule is calculated.
Weight of the hydrated salt = x g
Weight of the anhydrated salt= y g
Difference in weight = x-y gm
No of water molecule = x-y /18