Chemistry · General chemistry II · Concept
Solubility product (Ksp) and molar solubility
A slightly soluble ionic solid dissolves until its ions reach equilibrium with the solid. The solubility product, Ksp, is that equilibrium constant: the product of the ion concentrations, each raised to its coefficient. From Ksp you can find the molar solubility, see how a common ion lowers it, and predict whether mixing two solutions forms a precipitate.
The solubility equilibrium
For calcium fluoride, CaF₂(s) ⇌ Ca²⁺(aq) + 2F⁻(aq). The solid is left out of the expression, as pure solids always are, and the fluoride concentration is squared because of its coefficient.
Molar solubility
The molar solubility S is the number of moles of solid that dissolve per liter of solution. The ion concentrations follow from the formula: for CaF₂, [Ca²⁺] = S and [F⁻] = 2S, so Ksp = S(2S)² = 4S³.
Comparing solubilities
A smaller Ksp means a less soluble salt only when the salts release the same number of ions. When the ion counts differ, as for AgCl and CaF₂, compare molar solubilities instead.
The common-ion effect
A salt is less soluble in a solution that already contains one of its ions. The extra ion pushes the equilibrium back toward the solid, as Le Châtelier’s principle predicts.
Will a precipitate form?
Compute the ion product Q from the concentrations after mixing. If Q > Ksp, the solution is supersaturated and solid forms until Q falls to Ksp; if Q < Ksp, nothing precipitates; Q = Ksp describes a saturated solution.
Common mistakes
- Forgetting the coefficient: for CaF₂, [F⁻] = 2S, and it is squared, so Ksp = 4S³, not S³.
- Using concentrations from before mixing: once two solutions are combined, each ion is diluted by the total volume.
- Ranking salts that release different numbers of ions by Ksp instead of by molar solubility.
- Including the solid in the Ksp expression.
Key terms
- Solubility
- The most of a substance that can dissolve in a given amount of solvent at a given temperature. Molar solubility counts moles of the dissolved solid per liter, not moles of each ion.
- Solubility product
- Ksp, the equilibrium constant for a solid dissolving into its ions, such as [Ag⁺][Cl⁻] for AgCl. Comparing Q with Ksp tells you whether a precipitate will form.
- Common-ion effect
- Adding an ion that is already part of an equilibrium, such as Cl⁻ to a solution of AgCl, pushes the equilibrium back and lowers the solubility or ionization.
- Precipitation
- A solid forming out of a solution when its ions exceed the solubility, that is, when Q > Ksp. It may not appear at once if the crystals are slow to start.
- Precipitation reaction
- A reaction in which mixing two solutions forms an insoluble solid, the precipitate.
Work through an example
Silver chloride has Ksp = 1.8 × 10⁻¹⁰ at 25 °C. Find its molar solubility in water, in mol/L and in g/L.
Find the molar solubility from Ksp →Sources and scope
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