Question

The chemical composition of plaster of paris is:
A.$CaS{O_4}.2{H_2}O$
B.$CaS{O_4}.{H_2}O$
C.$CaS{O_4}.1/2{H_2}O$
D.$CaS{O_4}.3{H_2}O$

Answer 1

Hint: -At first think about the chemical formula of plaster of paris and the reactions where plaster of paris is used. Plaster of paris is a hydrate of calcium sulphate. It is produced when a calcium sulphate hydrate is heated.

Complete step by step answer:
Plaster of paris is made by heating the mineral gypsum. Large deposits were originally found outside of Paris in France. When gypsum is heated to about ${150^0}C$, it loses water and produces the powder, which is called plaster of paris.
$CaS{O_4}.2{H_2}O + heat \to CaS{O_4}.1/2{H_2}O + 3/2{H_2}O$
The chemical composition of plaster of paris is $CaS{O_4}.1/2{H_2}O$.

So the answer is C.

Additional Information:- Gypsum a soft sulphate mineral compound composed of calcium sulphate dihydrate. It is widely mined. It is used as a fertilizer and as the main constituent in many forms of plaster, black board and drywall. A lightly tinted variety of gypsum called alabaster has been used for sculpture by many cultures. It also forms as an evaporite mineral and as a hydration product of anhydrite. Gypsum is moderately water soluble. In contrast to many other salts, gypsum exhibits retrograde solubility. Gypsum occurs in nature as flattened and often twinned crystals and transparent , cleavable masses called selenite. Selenite contains no significant selenium. Selenite may also occur in a silky, fibrous form. Gypsum is a common mineral, with thick and extensive evaporate beds in association with sedimentary rocks. Gypsum is deposited from lake and sea water, as well as in hot springs from volcanic vapours.

Note:
Plaster of paris is also called calcium sulphate hemihydrate. It is used for making toys and decorative materials. It is also used for increasing smoothness of surfaces. Plaster of paris is used by doctors by supporting fractured bones in the right position.