Question

A) define water of crystillisation? B) give two examples having water of crystillisation.write their molecular formulae also.

Answer 1

the fixed number of water molecules present in 1 formula unit of salt is called water of crystallisation.
ex - CaSO4.5H2O 

Answer 2

water molecules forming an essential part of the crystal structure of some compounds is known as water of crystallisation.
Examples:- CuSo4.5H2O, Na2Al2SiO8.XH2O