Question

A solution contains 0.0350 M Ca2 and 0.0100 M Sr2 .What is the concentration of the first cation when the second starts to precipitate?

Answer 1

If the solution is precipitated by sulfate, then we need to calculate [SO₄^2-]

For CaSO₄, K_sp = 2.4 x 10^-5

For SrSO₄, K_sp = 3.44 x 10^-7

Now, SrSO₄ will precipitate when K_sp > 3.44 x 10^-7

Therefore, in a 0.0100 M Sr²⁺ (aq) solution SrSO₄ precipitates when

[SO₄^2-] > 3.44 x 10^-7 / 0.0100                                     (as K_sp = [Sr²⁺] [SO₄^2-])

or [SO₄^2-] > 3.44 x 10^-5 M

Now, solubility product for Ca²⁺ at this point = 3.44 x 10^-5 x 0.0350 = 0.12 x 10^-5

However, K_sp for CaSO₄ = 2.4 x 10^-5

thus solubility product for CaSO₄ is exceeded, so the SrSO₄ precipitates first

At this point, [Ca²⁺] = 2.4 x 10^-5/3.44 x 10^-5 M

[Ca²⁺] = 0.67 M