[Boxxxed]

Ba²⁺ and Sr²⁺ ions are in solution each with concentration of 0.10 M. Sodium sulfate is incrementally added to the solution. What will be the concentration of the first cation that precipitates when the second more soluble salt salt begins to precipitate?

ksp BaSO₄ = 1.1x10^-10

ksp SrSO₄ = 3.4x10^-7

SrSO₄ precipitates second.

Sulfide concentration right before/when SrSO₄ precipitates.

ksp = (Sr)(SO4)

SO4 = ksp / Sr

= (3.4x10^-7) / (0.10 M) = 3.4x10^-6

Concentration of Ba²⁺ when sulfide concentration is 3.4x10^-6

ksp = (Ba)(SO4)

Ba = ksp / SO4
    = (1.1x10^-10)/(3.4x10^-6) = 3.24x10^-5

Concentration of Ba²⁺ is 3.24x10^-5 when Sr²⁺ begins to precipitate.



The answer in the book is 3.9x10^-5. Why is my answer off?

[Borek]

Apart from the fact that you refer to sulfate "sulfide" for no apparent reason, your approach seems to be OK. The only thing I can think of is that you are expected to take solution ionic strength into account, but I have not checked if it makes sense.