Notice that each mole of barium sulphate dissolves to give 1 mole of barium ions and 1 mole of sulphate ions in solution.
This means: [Ba]=1.05×10moldm2+ [SO]=1.05×10moldm2− All we need to do is to put these values into the solubility product expression, and do the simple sum. Ksp=[Ba2+][SO2−4] =(1.05×10−5)×(1.05×10−5) =1.10×10−10mol2dm−6