Ionic EquilibriumHard

Question

A particular water sample has 136 ppm CaSO4. What percentage of water by mass must be evaporated in a container before solid CaSO4 begins to deposit. Assume that the solubility of CaSO4 does not change with temperature in the range 0oC to 100oC. The value of Ksp for CaSO4 = 1.6 × 10–5.

Options

A.80%
B.50%
C.75%
D.60%

Solution

$S = \sqrt{1.6 \times 10^{- 5}} = 4 \times 10^{- 3}\text{ M = }\frac{4 \times 10^{- 3} \times 136}{10^{3}} \times 10^{6} = 4 \times 136\text{ ppm}$

For increase in concentration 4 times, volume should be 1/4th. Hence, 75% water should be evaporated.

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