[Boxxxed]

When ksp=10^-10=[Ba2+][SO4^2-]

When Ba=[10^-3]
         SO4=[10^-2.3]
Q=10^-5.3 >>> 10^-10.0

So to find out how much precipitates wouldn't this be correct?

[10^-3-x][10^-2.3-x]=10^-10

Instead the pdf uses this method.

(x) (10^-2.3) = 10-10.0
x = 10^-7.7
BaSO4 precipitation = 10^-3 -10^-7.7 ~ 10-3 molL-1

It is saying that SO4 is constant because equilibrium is controlled by dissolution of a mineral with much higher ksp than barite. But shouldn't the first method work anyway?

[Borek]

It is saying that SO4 is constant because equilibrium is controlled by dissolution of a mineral with much higher ksp than barite. But shouldn't the first method work anyway?

How can the method work if it assumes concentration of SO₄^2- changes, while you are told it is constant?

[Boxxxed]

Because it has to change when barite precipitates? Its concentration must decrease equal to the amount of Ba that decreases. Ba doesn't just precipitate by itself, BaSO4 does.

[Borek]

It doesn't have to change - there is a SOURCE of SO₄^2- that keeps its concentration constant.