Question

The difference in the number of water molecules in gypsum and plaster of paris is:

• A. $\frac{1}{5}$
• B. $2$
• C. $\frac{1}{2}$
• D. $1\frac{1}{2}$

Answer 1

The correct option is D $1\frac{1}{2}$

Molecular formula of gypsum: $CaS{O}_4.2H_{2}O$

Molecular formula of plaster of paris: $CaS{O}_4.\frac{1}{2}{H}_{2}O$

So, difference between the number of water molecules
$=2-\frac{1}{2}=1\frac{1}{2}$​