Solubility Constant Calculator

Reviewed By: Patrick Myers (BSME)

Enter ion concentrations (for a 1:1 salt) or enter Ksp with the dissolution stoichiometry (molar solubility tab) to calculate the solubility product constant (Ksp). Ksp describes the equilibrium between a sparingly soluble solid and its dissolved ions at a specified temperature.

Basic Ksp (1:1)

Molar Solubility (a:b)

For a 1:1 salt (AB), enter any 2 values to calculate the missing variable. Uses: Ksp = [A][B].

Select a common 1:1 salt (optional, values typically at 25 °C):
Ion 1 concentration, [A]

Ion 2 concentration, [B]

Solubility Product Constant (Ksp)

Solubility Constant Formula

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

A_aB_b(s)\rightleftharpoons aA + bB

The Ksp expression is:

K_{sp} = [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).

Write the balanced dissolution reaction for the solid (for example, A_aB_b(s) ⇌ aA + bB).
Write the Ksp expression using the stoichiometric coefficients as exponents: Ksp = [A]^a[B]^b.
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).
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