Solubility Product (Ksp) Calculator
Compute the solubility product Ksp of a sparingly soluble salt from its molar solubility and its dissolution stoichiometry.
How it works
For AₓBᵧ ⇌ x·Aⁿ⁺ + y·Bᵐ⁻ : Ksp = (x·s)ˣ · (y·s)ʸ
The solubility product Ksp is the equilibrium constant for a slightly soluble ionic solid dissolving into its ions. Because each ion’s concentration depends on how many of that ion each formula unit releases, the exponents in the Ksp expression come straight from the dissolution stoichiometry. Given the molar solubility s — the moles that dissolve per litre — you substitute the ion concentrations and multiply.
Worked example
For CaF₂ (type AB₂) with molar solubility s = 2.1 × 10⁻⁴ mol/L: [Ca²⁺] = s and [F⁻] = 2s, so Ksp = s·(2s)² = 4s³ = 4 × (2.1 × 10⁻⁴)³ ≈ 3.7 × 10⁻¹¹.
Sources: Solubility equilibrium — Wikipedia
Frequently asked questions
What is molar solubility?
It is the number of moles of the solid that dissolve per litre of solution to reach saturation. It is the s that this calculator turns into a Ksp.
Why does CaF₂ use 4s³ instead of s²?
Each formula unit releases one Ca²⁺ and two F⁻, so [F⁻] = 2s. Ksp = [Ca²⁺][F⁻]² = s·(2s)² = 4s³. The coefficients become both multipliers and exponents.
How do I go from Ksp back to solubility?
Invert the expression for the given salt type. For an AB salt, s = √Ksp; for AB₂, s = ∛(Ksp/4). The common-ion calculator handles the case where another source of the ion is present.
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This calculator is for educational purposes. Double-check important results before acting on them.