After 20.0g of Na2SO4 is added to a 0.5L saturated solution of CaSO4, does the concentration of Ca2+ increase, decrease, or stay the same? Assume that the overall volume of the solution does not change.

Answer 1

.......CaSO4 --> Ca^2+ + SO4^2-
I......solid......0.......0
C......solid......x.......x
E......solid......x.......x

Ksp CaSO4 = (Ca^2+)(SO4^2-)
Now if we add a large excess of sulfate as Na2SO4, that makes SO4^2- very large, Le Chatelier's Principle says the reaction has too much sulfate so the reaction shifts to the left. That decreases (Ca^2+).