Question

Plaster of Paris in contact with water sets to a hard mass whose composition is:
A.$CaS{O_4}$
B.$CaS{O_4}.{H_2}O$
C.$CaS{O_4}.2{H_2}O$
D.$CaS{O_4}.Ca{\left( {OH} \right)_2}$

Answer 1

Hint:Plaster of Paris is commonly known as (POP). Plaster of Paris (Calcium sulfate) is not water-soluble. So, when the plaster of Paris comes in contact with water it sets to a hard mass. Here we will determine the composition of that hard mass formed when plaster of Paris comes in contact with water.

Complete answer:
First, we will know about the chemical formula of the plaster of Paris, its chemical name, and how it is prepared.
The chemical formula of the plaster of Paris is $CaS{O_4}.\dfrac{1}{2}{H_2}O$ . It is also known as hemihydrate of calcium sulfate. It is obtained when gypsum $CaS{O_4}.2{H_2}O$ is heated to $393K$ .
$CaS{O_4}.2{H_2}O\xrightarrow{\Delta }2CaS{O_4}.\dfrac{1}{2}{H_2}O + \dfrac{3}{2}{H_2}O$
Gypsum is mainly found in Paris. Due to this fact hemihydrate of calcium sulfate is known as Plaster of Paris.
It is noted that the Plaster of Paris reflects a different property of setting with water. On mixing a sufficient amount of water it forms a plastic that gets converted into a hard solid in a few minutes.
The reaction of plaster of Paris with water can be written as given below,
$CaS{O_4}.\dfrac{1}{2}{H_2}O + \dfrac{3}{2}{H_2}O \to CaS{O_4}.2{H_2}O$
From the above chemical reaction, we can observe that when the plaster of Paris reacts with water it forms ‘gypsum’ whose chemical composition is $CaS{O_4}.2{H_2}O$. On mixing with water Plaster of Paris forms a hard mass called ‘gypsum’ which is the final product of the setting of plaster of Paris.

Hence, the correct option is (C).
Note:
The hard mass formed ‘gypsum’ is also called monoclinic calcium sulfate dihydrate with chemical composition $CaS{O_4}.2{H_2}O$ .
It is observed that the largest use of plaster of Paris is in the building industry as well as plasters.Plaster of Paris is also classified as a hazardous substance.
It is also used in the manufacture of statues and toys.
The addition of alum to plaster of Paris makes the setting very hard and the mixture formed is known as Keene’s mixture.