Question

The number of water of crystallisation present in one molecule of Plaster of Paris is/are:

• A. 4.0
• B. 1.0
• C. 0.2
• D. 0.5

Answer 1

The correct option is D 0.5

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt. The chemical formula of Plaster of Paris is ${\text{CaSO}}_4.\frac{1}{2}{\text{H}}_2\text{O}$. In fact, Plaster of Paris is also called calcium sulphate hemihydrate.
Thus, the number of molecules of water of crystallisation in one molecule of Plaster of Paris is 0.5.