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Solubility Product (Ksp) Calculator

Enter the molar solubility and ion coefficients to find Ksp.

How it is calculated

For a salt that dissolves as MaXb → a M + b X, the tool takes the molar solubility s (mol/L) and the two stoichiometric coefficients a and b. It first finds the ion concentrations, [M] = a·s and [X] = b·s, and then applies Ksp = [M]a × [X]b = (a·s)a × (b·s)b. All three results are printed in scientific notation with four decimals, the two concentrations in mol/L (M). Ksp itself is dimensionless in the equilibrium-constant sense, so the tool shows no unit for it. Inputs with s below 0, or a coefficient that is below 1, above 20 or not a whole number, give no result. The calculation assumes a pure saturated solution, no common ion and no activity corrections, so values from real, salty solutions will differ.

Worked example

Take a salt of the MX2 type, so a = 1 and b = 2, with a molar solubility of s = 0.0002 mol/L. The cation concentration is 1 × 0.0002 = 2.0000 × 10⁻⁴ M and the anion concentration is 2 × 0.0002 = 4.0000 × 10⁻⁴ M. Then Ksp = (2.0000 × 10⁻⁴)1 × (4.0000 × 10⁻⁴)2 = 2.0000 × 10⁻⁴ × 1.6000 × 10⁻⁷, which the tool prints as 3.2000 × 10⁻¹¹.

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Common mistakes

  • Entering the concentration of an ion as s. The input is the amount of salt that dissolves per litre; the tool multiplies by the coefficients itself, so typing 2s for a 1:2 salt counts the factor twice.
  • Swapping a and b. For Ca3(PO4)2 the cation coefficient is 3 and the anion coefficient is 2; reversing them gives a different Ksp, because the exponents change too.
  • Giving solubility in g/L. The tool needs mol/L, so divide the mass solubility by the molar mass first.
  • Typing 0 as a coefficient for a missing ion. Both coefficients must be at least 1, otherwise nothing is calculated.
  • Using the result to compare salts with different a + b. Ksp values only compare directly for salts of the same formula type.

About this calculator

The solubility product constant (Ksp) describes the equilibrium between a sparingly soluble ionic salt and its dissolved ions: for a salt MₐXᵦ that dissociates as MₐXᵦ ⇌ aMᵇ⁺ + bXᵃ⁻, Ksp = [M]ᵃ[X]ᵇ at saturation, where the ion concentrations are expressed in terms of the molar solubility s using the stoichiometric coefficients (e.g., [M] = as, [X] = bs).

This calculator takes the salt's molar solubility and its ion coefficients and computes Ksp directly, which is the reverse of the more common textbook problem of finding solubility from a known Ksp. Chemistry students and analytical/environmental chemists use it to characterize precipitation reactions, predict whether a salt will dissolve or precipitate under given ion concentrations, and interpret solubility data for applications like water hardness, mineral scaling, and qualitative-analysis lab work.

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