Question

A 75.0 mL sample of 1.56×10−2 M Na2SO4 is added to 75.0 mL of 1.25×10−2 M Ca(NO3)2. What percent of the Ca 2+ remains unprecipitated?

Answer 1

Calculate initial concentrations of both Ca2+ and SO42-. When CaSO4 precipitates both concentrations go down by the same amount (because of the stoichiometry). They go down till their product hits Ksp. Try to express it as an equation - with one unknown (amount of precipitate).