Solubility equilibria – Ksp, the common-ion effect and pH
ChemistryKinetics & EquilibriumAges 16–17
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Sign in to playA saturated solution of a sparingly soluble salt (AgCl, PbI₂, Ca(OH)₂, Mg(OH)₂, CaCO₃, BaSO₄), with a magnified view of the dynamic equilibrium between the crystal and its ions. Work out the molar solubility from Ksp with the correct stoichiometry, add water or more solid, dissolve the salt in a solution with a common ion or in a pH buffer, and follow the change on a solubility graph. On the mixing screen, compare Q with Ksp to predict whether a precipitate forms and calculate its mass.
Lesson: Solubility equilibria – the solubility product Ksp, molar solubility, the common-ion effect, the effect of pH and predicting precipitation
What it shows
In a saturated solution, a sparingly soluble ionic solid is in dynamic equilibrium with its ions: ions leave the crystal as fast as they rejoin it. The solubility product Ksp is the product of the ion concentrations, each raised to its coefficient, so the molar solubility depends on the formula: s = √Ksp for AgCl but s = ∛(Ksp/4) for PbI₂. Adding a common ion pushes the equilibrium back and lowers the solubility. Lowering the pH dissolves hydroxides and carbonates, because H⁺ removes OH⁻ or CO₃²⁻. When two solutions are mixed, a precipitate forms only if the ion product Q exceeds Ksp.
How to use
Choose a Salt. On the Saturated solution tab, set the Volume of liquid and the Solid added, or use + 50 mL liquid and + 0.1 g solid. Under Dissolve in, choose a solution with a common ion and set its concentration. Tick Keep pH constant (buffer) and move pH, then switch Graph to compare. On the Mixing solutions tab, set the concentration and volume of each solution, predict from the Q and Ksp scale, then press Mix. Each mixture is recorded in the table.
Parameters you can change
- Screen Saturated solution, Mixing two solutions
- Sparingly soluble salt AgCl (silver chloride), PbI₂ (lead(II) iodide), Ca(OH)₂ (calcium hydroxide), Mg(OH)₂ (magnesium hydroxide), CaCO₃ (calcium carbonate), BaSO₄ (barium sulfate)
- Liquid the salt is dissolved in Distilled water, Solution containing the common cation, Solution containing the common anion
- Concentration of the common ion in the liquid 1e-06–1 mol/L
- Volume of liquid 50–1000 mL
- Mass of solid salt added 0.01–5 g
- Keep the pH constant with a buffer
- pH of the buffer 0–14
- Concentration of the cation solution (mixing) 1e-06–1 mol/L
- Concentration of the anion solution (mixing) 1e-06–1 mol/L
- Volume of the cation solution 10–100 mL
- Volume of the anion solution 10–100 mL
Questions to explore
- PbI₂ has a larger Ksp than CaCO₃. Is it also more soluble, and why is the calculation different?
- How many times less soluble is AgCl in 0.01 mol/L sodium chloride than in pure water?
- Why do Mg(OH)₂ and CaCO₃ dissolve in acid while AgCl does not?
