Question

Ksp fo CaSO4 is 6.1*10?-5. how many grams of CaCl2 must be added to 500ml of 0.010M H2SO4 to just start preciptation of CaSO4?

I cant figure this out at all.

Answer 1

I would do this.
For a ppt to form you know Ksp must be exceeded; therefore,
..........CaSO4(s) ==> Ca^2+ + SO4^2-
E.........solid........x........x

You also know that adding 0.01M H2SO4 will increase the solubility of CaSO4 due to the reaction of SO4^2- + H^+ ==> HSO4^-.

You work this the same was as you did the CaCO3 problem.
solubility CaSO4 = (Ca^+) = S
S = SO4^2- + HSO4^-
For HSO4^-, use k2 and solve for (HSO4^-) = some number x SO4^2-.
Plug into S = SO4 + HSO4 which makes
S = some number x SO4
Then SO4 = S/some number and that goes into the Ksp expression. Solve for S = Ca^2+ which gives M (mols/L), take half to convert to 500 mL, and convert to grams CaCl2.