Question

The chemical formula of Plaster of Paris is:
A. CaSO$_4$
B. CaSO$_4$.2H$_2$O
C. CaSO$_4$.$\dfrac{1}{2}$H$_2$O
D. CaSO$_4$.H$_2$O

Answer 1

Hint: The Plaster of Paris is composed of calcium ion, and sulphate ion, and water molecules are too attached with it. It is also known as calcium sulphate hemihydrate, and is of white colour. It is used to make sculptures, and metal castings, and also used as coating on wood to avoid fire hazards.

Complete step by step answer:
1. First, we know that the Plaster of Paris is a compound of mainly calcium. It is formed by heating gypsum at about 150$^{\circ}$C.
2. When we heat the gypsum, it loses water, and produces the powder form of compound i.e. Plaster of Paris, in this reaction heat is required to break as gypsum is a hard solid compound.
3. Now, we can write it as Gypsum + Heat $\rightarrow$ Plaster of Paris + steam. The chemical reaction of heating gypsum is 2CaSO$_4$.H$_2$O + Heat $\rightarrow$ 2CaSO$_4$.$\dfrac{1}{2}$H$_2$O +3H$_2$O, CaSO$_4$.H$_2$O represents the compound gypsum.
4. In this reaction CaSO$_4$.$\dfrac{1}{2}$H$_2$O is the compound formed, and it is known to be Plaster of Paris, thus we can say that the Plaster of Paris is a compound of calcium, sulphate, and water molecules.
5. Therefore, we can see that there is presence of two molecules of water around calcium sulphate.
Thus, the chemical formula is CaSO$_4$.$\dfrac{1}{2}$H$_2$O. The correct option is C.

Note: Don’t get confused between the no. of water molecules present in the Plaster of Paris. Just write down the proper chemical reaction of heating a gypsum and balance it, we will get the chemical formula of it.