Ionic EquilibriumHard

Question

The solubility product of CaF2 is 1.08 × 10−10. What mass of CaF2 will dissolve in 500 ml water in order to make a saturated solution? (Ca = 40, F = 19)

Options

A.3 × 10−4 g
B.1.17 × 10−2 g
C.1.17 mg
D.3 × 10−3 g

Solution

For CaF2: Ksp = 4S3

$\Rightarrow S = \left( \frac{K_{sp}}{4} \right)^{1/3} = \left( \frac{1.08 \times 10^{- 10}}{4} \right)^{1/3} = 3 \times 10^{- 4}\text{ M}$

∴ Mass of CaF2 needed = $\left( \frac{500 \times 3 \times 10^{- 4}}{1000} \right) \times 78 = 1.17 \times 10^{- 2}\text{ gm}$

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