If a solution contains calcium ions (0.27 M) and barium ions (0.070 M), what concentration of SO42- would be required to selectively precipitate the barium ions from solution? BaSO4 will precipitate and CaSO4 will not precipitate if [SO42 Solution BaSO4 Ksp=1.1*10-10 [ba+][so42-]=1.1*10-10 [so42-]=1.1*10-10/0.070=1.57*10-9 M CaSO4 Ksp=4.93.