Question

What is the percentage composition of water in gypsum salt?

• A. 20.93%
• B. 21.93%
• C. 22.52%
• D. 23.60%

Answer 1

The correct option is A 20.93%

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

$\therefore$ Molecular mass of gypsum = Molecular mass of $CaS{O}_4$ + 2 $\times$ Molecular mass of $H_{2}O$
= (1 $\times$ 40) + (1 $\times$ 32) + (4$\times$ 16) + (2$\times$ 1) + (1$\times$ 16)
= 172u

Out of the 172u, (2 $\times$ 18)u is contributed by water.

$\therefore$ Mass % of water = $\frac{36}{172}$ $\times$ 100
= 20.93%