Question

Calcium hydroxide and calcium sulfate are both used to prepare a saturated solution, S. Both

solids are only sparingly soluble, and the pH of S is found to be 11.6.

(i) Given that the Ksp for calcium hydroxide is 4.0 x 10-5

and the Ksp for calcium sulfate is

2.5 x 10-5

, determine the concentration of sulfate ions present in S.

Answer 1

Solution.

$pH + pOH = 14$

$pOH = 14 - pH$

pOH = 2.4

$[OH^-] = 10^{-pOH}$

$[OH^-] = 3.98 \times 10^{-3} M$

$[Ca^{2+}][OH^-]^2 = Ksp(Ca(OH)2)$

$[Ca^{2+}] = (\frac{Ksp(CaOH)2}{[OH^-]^2}) = 2.53 M$

$[Ca^{2+}][SO4^{2-}] = Ksp(CaSO4)$

$[SO4^{2-}] = \frac{Ksp(CaSO4)}{[Ca^{2+}]} = 9.88 \times 10^{-6} M$

Answer:

$[SO4^{2-}] = 9.88 \times 10^{-6} M$