A particular water sample has . What percentage of water, by mass, must be evaporated in a container — Ionic Equilibrium Chemistry Question
Question
A particular water sample has $136 \text{ ppm} \text{ CaSO}_4$. What percentage of water, by mass, must be evaporated in a container before solid $\text{CaSO}_4$ begins to deposit. Assume that the solubility of $\text{CaSO}_4$ does not change with temperature in the range $0^\circ\text{C}$ to $100^\circ\text{C}$. $K_{sp}$ of $\text{CaSO}_4 = 1.6 \times 10^{-5}$.
💡 Solution & Explanation
$136 \text{ ppm}$ of $\text{CaSO}_4$ implies $136 \text{ mg/L}$. The molar mass is $136 \text{ g/mol}$, so the initial concentration is $10^{-3} \text{ M}$. Precipitation of $\text{CaSO}_4$ will begin when the ionic product equals $K_{sp}$, meaning $[\text{Ca}^{2+}] = \sqrt{1.6 \times 10^{-5}} = 4.0 \times 10^{-3} \text{ M}$. To increase the concentration from $1 \times 10^{-3} \text{ M}$ to $4 \times 10^{-3} \text{ M}$, the volume of the solution must be reduced to $\frac{1}{4}$ of its initial volume. Thus, $\frac{3}{4}$ (or 75%) of the water must be evaporated. Therefore, correct answer is C.