Ksp Solubility Calculator – Free Calculator

Understanding the Ksp solubility calculator — concept and principle

The solubility product constant, Ksp, describes the equilibrium between a sparingly soluble ionic solid and its dissolved ions. From Ksp you can work out how much of the salt actually dissolves in pure water. This calculator converts a Ksp value and the formula's ion coefficients into molar solubility (mol/L) and mass solubility (mg/L). It is used in general chemistry, precipitation and gravimetric analysis, and water chemistry.

Each dissolved AgCl gives one Ag⁺ and one Cl⁻, so s² = Ksp and only about 1.92 mg dissolves per litre.

How it is calculated

For a salt AₐBᵦ that dissolves as AₐBᵦ(s) ⇌ aA + bB:

Ksp = [A]^a [B]^b = (a·s)^a (b·s)^b, where s is the molar solubility.
Solving for s: s = (Ksp ÷ (a^a × b^b))^(1/(a + b)).
Mass solubility in mg/L = s × M × 1000, with M the molar mass in g/mol.

Coefficient a belongs to the cation and b to the anion.

The principle

When s mol of solid dissolves per litre, it releases a·s mol/L of cation and b·s mol/L of anion. The activity of a pure solid is 1, so it does not appear in the equilibrium expression. Substituting these concentrations into Ksp gives one equation in one unknown. Because the coefficients appear as exponents, salts with different formulas cannot be ranked by Ksp alone; compare their s values instead.

Worked example

Defaults: silver chloride, AgCl, Ksp = 1.8 × 10⁻¹⁰, a = 1, b = 1, M = 143.32 g/mol.

a^a × b^b = 1 × 1 = 1.
s = (1.8 × 10⁻¹⁰)^(1/2) = 0.0000134164 mol/L (1.34 × 10⁻⁵ M).
Mass solubility: 1.34164 × 10⁻⁵ × 143.32 × 1000 = 1.922839575 mg/L.

Each point is the calculator re-run with only “Cation coefficient” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Cation coefficient	Molar solubility (mol/L)
0.25	2.111689e-8
0.5	4.016598e-7
0.75	0.0000030558
1	0.0000134164
1.25	0.000041226
1.5	0.0000991869
2	0.0003556893

How to read the result

About 1.9 mg of AgCl dissolves per litre of pure water at 25 °C, which is why it precipitates readily in chloride tests. For a 1:2 salt like CaF₂ (Ksp about 3.9 × 10⁻¹¹), s = (Ksp ÷ 4)^(1/3) ≈ 2.1 × 10⁻⁴ M, so it is more soluble than AgCl even though its Ksp is smaller. If the solution already contains a common ion (for example NaCl for AgCl), the solubility is much lower than this calculation gives.

Common mistakes

Forgetting the coefficients in front of s, for example writing Ksp = s × s² instead of s × (2s)² for CaF₂.
Comparing Ksp values directly for salts with different ion ratios.
Using a Ksp from a different temperature.
Applying the pure-water result to seawater or solutions with common ions or complexing agents.

Keep in mind: The calculation assumes ideal behaviour in pure water: no common ions, no complex formation, no hydrolysis of the ions and activity coefficients equal to 1. Real solubilities can differ significantly, and published Ksp values vary between sources.

Frequently asked questions

Does a larger Ksp always mean a more soluble salt?

Only when the salts have the same ion ratio, such as comparing two 1:1 salts.

How does a common ion change solubility?

Adding Cl⁻ to AgCl pushes the equilibrium back towards the solid; in 0.1 M Cl⁻, s ≈ Ksp ÷ 0.1 = 1.8 × 10⁻⁹ M.

How do I enter Ag₂CrO₄?

Use a = 2 for Ag⁺ and b = 1 for CrO₄²⁻.

Sources: Harris, D. C., Quantitative Chemical Analysis, W. H. Freeman · Haynes, W. M. (ed.), CRC Handbook of Chemistry and Physics, CRC Press

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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