Calculate Ksp, ion concentrations, or molar solubility for salts from concentration values, stoichiometric coefficients, and common 25°C Ksp reference data. Select the quantity to solve; enter the displayed required fields. The molar-solubility modes assume no added common ions and no coupled reactions.

Dilute-solution approximation using concentrations in mol/L. Ksp requires equilibrium concentrations at a fixed temperature. Pure-water solubility excludes common ions, complexation and other coupled reactions.

Solubility Constant Formula

The solubility product constant is written from the balanced dissolution reaction. For a salt that dissolves as:

AₐBb(s)leftharpoons aA + bB

The dilute concentration approximation to the activity-based Ksp expression is:

Kₛₚ = [A]a[B]b

For a 1:1 salt AB(s) ⇌ A + B, this becomes Ksp = [A][B]. In pure water (no added common ions) the molar solubility is S and [A]=[B]=S, so Ksp = S² (ignoring activity effects).

Variables:

  • Ksp is the solubility product constant (defined for a specific dissolution reaction and temperature).
  • [A] and [B] are the equilibrium molar concentrations of the dissolved ions (mol/L).
  • a and b are the stoichiometric coefficients from the balanced dissolution equation (and also the exponents in the Ksp expression).
  • S is the molar solubility (mol/L) of the solid in pure water (when applicable).

What is a Solubility Constant?

The solubility constant, also known as the solubility product constant (Ksp), describes the equilibrium between a sparingly soluble ionic solid and its dissolved ions. It is a constant for a given substance at a specific temperature and is used to estimate solubility and predict whether a precipitate will form (via comparison of the ion product Q to Ksp).

How to Calculate Solubility Constant?

The following steps outline how to calculate the Solubility Product Constant (Ksp).

  1. Write the balanced dissolution reaction for the solid (for example, AaBb(s) ⇌ aA + bB).
  2. Write the Ksp expression using the stoichiometric coefficients as exponents: Ksp = [A]a[B]b.
  3. Determine the equilibrium ion concentrations. In pure water, these can often be written in terms of molar solubility S (e.g., [A]=aS and [B]=bS).
  4. Substitute the equilibrium concentrations into the Ksp expression and solve for Ksp (or solve for S if Ksp is known).

Example Problem :

Silver chloride dissolves as AgCl(s) ⇌ Ag+ + Cl−. If its molar solubility in pure water is S = 1.33×10−5 mol/L, then:

Ksp = S² = (1.33×10−5)² ≈ 1.77×10−10