Precipitation Problem?

Question

A 100.-mL sample of solution contains 10.0 mmol of Ca2+ ion. How many mmol of solid Na2SO4 must be added in order to cause precipitaton of 99.9% of the calcium as CaSO4? The Ksp of CaSO4 is 6.1 x 10^-5. Assume the volume remains constant.

Please help, I don't know the steps to do these kinds of problems.

Answer 1

100.mL solution contains 10.0 mmol of Ca2+ ion <==> 0.1M Ca2+
==> Ksp = 6.1 x 10^-5 = [Ca2+]*[SO4--] ==> [SO4--] = 6.1 x 10^-4 M ==> 6.1 x 10^-3 mol in 100mL <==> 6.1 mmol of SO4-- in 100mL.