Question

The molecular formula of gypsum is
(A). $\text{CaS}{{\text{O}}_{4}}.{{\text{H}}_{2}}\text{O}$
(B). $\text{CaS}{{\text{O}}_{4}}.2{{\text{H}}_{2}}\text{O}$
(C). $\text{2CaS}{{\text{O}}_{4}}.{{\text{H}}_{2}}\text{O}$
(D). $\text{CaS}{{\text{O}}_{4}}.3{{\text{H}}_{2}}\text{O}$

Answer 1

- Hint: Gypsum is the hydrated calcium sulphate normally found in nature. It is an important raw material in the manufacture of Plaster of Paris ($\text{CaS}{{\text{O}}_{4}}.\dfrac{1}{2}{{\text{H}}_{2}}\text{O}$). Varieties of gypsum known as “satin spar” and “alabaster” are used for a variety of ornamental purposes.

Complete step-by-step solution -
We all have seen beautiful designs made on the ceiling and walls of rooms in many houses. They are made with Plaster of Paris, also called POP.
And Plaster of Paris is made of Gypsum. It is a mineral which is not toxic in nature and a sulfate compound. It is chemically known as calcium sulfate dihydrate. It consists of water, calcium sulfate attached to oxygen. The chemical formula of gypsum is $\text{CaS}{{\text{O}}_{4}}.2{{\text{H}}_{2}}\text{O}$ .
Additional Information:
Plaster of Paris (POP) is manufactured from gypsum. When gypsum is heated at about 325K, it loses part of its water of crystallization to form $\text{CaS}{{\text{O}}_{4}}.\dfrac{1}{2}{{\text{H}}_{2}}\text{O}$ which is called plaster of paris. When made into a paste with a little water, Plaster of Paris sets to a hard mass, which expands with hardening. It should also be noted that on mixing with water, it changes to gypsum once again giving a hard solid mass
$\text{CaS}{{\text{O}}_{4}}.\dfrac{1}{2}{{\text{H}}_{2}}\text{O+1}\dfrac{1}{2}{{\text{H}}_{2}}\text{O}\to \text{CaS}{{\text{O}}_{4}}.2{{\text{H}}_{2}}\text{O}$

Note: Plaster of Paris finds use in making casts and patterns. It is used for making plaster casts to hold fractured bones in position while they set. It is also used for making chalks for writing on blackboard. Nowadays it is increasingly being used for plastering the walls, pillars and ceilings and to make ornamental patterns on them.