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

Ca(OH)₂ --> Ca²⁺ + 2OH^-;

pOH=14 - pH=14-11.6=2.4;

pOH=-lg[OH^-];

[OH^-]=10^-pOH=10^-2.4=0.00398≈4*10^-3 mol/L;

[Ca²⁺]=[OH^-]/2=2*10^-3 mol/L;

[SO₄^2-]=[Ca2+]=2*10^-3

Answer: 2*10^-3