Question

The difference of water molecules in gypsum and plaster of Paris is:
A.) $\dfrac{5}{2}$
B.) $2$
C.) $\dfrac{1}{2}$
D.) $1\dfrac{1}{2}$

Answer 1

Hint: In this question, the plaster of Paris has the chemical formula which includes half a molecule of water in it and on the other hand gypsum has the chemical formula which includes two molecules of water in it.

Complete step by step answer:
In this question, Plaster of Paris have the chemical name as calcium sulphate hemihydrate which
contains one calcium sulphate and half molecule of water. The short form of plaster of Paris is $POP$. It has a chemical formula as $CaS{O_4}.\dfrac{1}{2}{H_2}O$. The plaster of Paris got this name because of
its preparation from the abundant gypsum found near Paris. The plaster of Paris is prepared
from gypsum when gypsum is heated at the temperature of $373K$. The plaster of Paris is a
white powder which hardens when moistened and allowed to dry. It is used to make sculptures
and metal castings used as decorative in buildings. It is also used as fire resistant.
On the other hand, gypsum gave the chemical name as calcium sulphate dihydrate which
include contains one calcium sulphate and two molecules of water. Gypsum has the chemical
formula as $CaS{O_4}.2{H_2}O$. It is used to make plaster of Paris. It is also used in construction, tile manufacturing and in the cement industry.
As we know that gypsum has two molecules of water and plaster of Paris has half molecules of water. Therefore,
Difference in molecule of water in gypsum and plaster of Paris:
 $
   = 2 - \dfrac{1}{2} \\
   = \dfrac{3}{2} = 1\dfrac{1}{2} \\
 $

Hence, option D.) is the correct answer.

Note: Always remember that the plaster of Paris that is known as $POP$ is prepared when we heat the gypsum at the temperature $373K$. This is why plaster of Paris ($POP$) has less molecules of water that is half and on the other hand gypsum has more molecules of water that is two.