HSC · Topic explainer
Predicting precipitation from Q and Ksp
Calculate the ion product Q using ion concentrations immediately after mixing. Use the same powers as in the solubility product expression. If Q is greater than Ksp, precipitation is favoured. If Q is smaller, the solution is unsaturated with respect to that solid.
Worked example
After mixing, suppose [Ag⁺] is 1.0 × 10⁻⁴ mol/L and [Cl⁻] is 2.0 × 10⁻⁴ mol/L. The ion product for AgCl is 2.0 × 10⁻⁸.
If Ksp is given as 1.8 × 10⁻¹⁰, Q exceeds Ksp and AgCl can precipitate. When testing a second possible solid, consider whether precipitation of the first changes an ion concentration used in the second calculation.
Official exam practice
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