[LHM]

The K_sp for BaSO₄ is 1.1*10^-10 and that for BaSeO₄ is 2.8*10^-11. A 1.0 M solution of BaCl₂ is added slowly to a solution that is 1.0*10^-4 M in sodium sulfate, Na₂SO₄ and 1.0*10^-4 M in sodium selenate Na₂SeO₄. What percentage of one anion has precipitated at the point which the second anion just begins to precipitate?
A) Under 10%
B) About 25%
C) About 75%
D) Over 90%

So I have that the concentration at which BaSO₄ precipitates is 1.0*10^-5, and the concentration at which BaSeO₄ precipitates is 5.3*10^-6. After that, what are you supposed to do?

[Borek]

What is concentration of Ba²⁺ when the second anion starts to precipitate? What must be concentration of the FIRST anion at the same moment?

[LHM]

What is concentration of Ba²⁺ when the second anion starts to precipitate? What must be concentration of the FIRST anion at the same moment?

Is the concentration of the Ba²⁺ 1.0*10^-5? How do you figure out the concentration of the first anion, does it not have anything to do with the rate?

[Borek]

It has nothing to do with rate, it is a direct application of Ksp.

Is the concentration of the Ba²⁺ 1.0*10^-5?

No. You know concentration of SO₄^2-, use it to calculate what concentration of Ba²⁺ is necessary for BaSO₄ precipitation.

[LHM]

Would the concentration of Ba²⁺ be (1.1*10^-10)/(1.0*10^-4)=1.1*10^-6 when the second anion starts to precipitate? And the concentration of the first anion at the same time would be (2.8*10^-11)/(1.1*10^-6)=2.5*10^-5?

[Borek]

Wasn't that hard 

[LHM]

So to get the percent that has precipitated, do you take 2.5*10^-5, divide it by 1.0*10^-4, and multiply by 100? That gives 25%, which is B?

[Borek]

Finishing the question is up to you