What is the molecular formula of Gypsum?

The molecular formula of Gypsum is $\mathit{CaS}{O}_4.2H_{2}O$.
 

Since the plaster of Paris is calcium sulphate which has the formula CaSO4.½H2O, it readily absorbs moisture(water) from the environment. If water of crystallization is added to it, it forms gypsum material, and its reaction is given as:

CaSO₄.1/2H₂O (plaster of Paris) + 3/2 H₂O(water) $\to$ CaSO₄.2H₂O(gypsum)