Question

A 50.0 −mL sample of 1.50×10−2 M Na2SO4(aq) is added to 50.0 mL of 1.28×10−2 M Ca(NO3)2(aq). What percentage of the Ca2+ remains unprecipitated?

Answer 1

When we have Ksp CaSO4 = 9.1x10^-6 so by substitution in Ksp formula:
Ksp CaSO4 = [Ca+2][SO4] 
1.9x10^-6   = [Ca+2][1.5x10^-2]
∴ [Ca+2] M in the solution =1.27x10^-4
when percent remaining = [Ca in solution]/[C original value] * 100
                                          = [(1.27x10^-4*(50ml+50ml)]/[1.28x10^-2*50ml]*100
                                          ≈ 2%