Question

$K_{sp}$ of $CaSO_4\cdot5H_2O$ is $9 \times 10^{-6}$, find the volume for $1 \,g$ of $CaSO_4$ (M.wt. = 136).

• A. 2.45 litre
• B. 5.1 litre
• C. 4.52 litre
• D. 3.2 litre

Answer 1

Answer: (A)

$CaSO_{4} (s) \rightleftharpoons \underset{S}{Ca^{2+}}(aq) +\underset{S}{SO^{2-}_{4}}(aq)$

$K_{s p}=S^{2}=9 \times 10^{-6} $

$S=3 \times 10^{-3} \,mol \,L ^{-1} $

Solubility in $g \, litre ^{-1}$ molecular mass $\times S$

$=136 \times 3 \times 10^{-3}=408 \times 10^{-3} g \,L ^{-1}$

$408 \times 10^{-3} \,g$ of $CaSO _{4}$ present in 1 litre

$1 \,g$ of $CaSO _{4}$ present is $\frac{1}{408 \times 10^{-3}}=2.45$ litre