Question

Calcium sulphate, $CaS{O}_4$ is a white crystalline substance. Gypsum is a mineral or natural substance that is hydrate of calcium sulphate. $1.032$ g sample of gypsum contains $0.816$ g $CaS{O}_4$. How many moles of water are contained in gypsum per mole of $CaS{O}_4$. Give your answer in nearest integer. ($Ca=40$, $S=32$)

• A. $6$
• B. $3$
• C. $2$
• D. $5$

Answer 1

Answer: (C)

$2$

$1.032$ g of gypsum contains $0.816$ g of calcium sulphate.

Hence, $136.14$ g of calcium sulfate will be present in $\frac{1.032}{0.816}\times 136.14=172.17$ g.

This is the molecular weight of gypsum.

Weight of water present in $1$ mole of gypsum $=172.17-136.14=36$ g.

This corresponds to $2$ moles of water.