Solubility product and molar solubility
The solubility product is the equilibrium-constant expression for dissolution of a sparingly soluble ionic solid, with each dissolved-ion term raised to its stoichiometric coefficient.
For a salt MₐXᵦ with molar solubility s, the ideal concentrations are as and bs when no common ions or other reactions alter the equilibrium.
How to use the Ksp calculator
- Select a direction: Calculate Ksp or calculate molar solubility.
- Enter the known value: Provide positive molar solubility or Ksp.
- Enter coefficients: Use positive whole-number coefficients from the balanced dissolution equation.
- Calculate: Interpret the result within the ideal-equilibrium assumptions.
Formula and variables
The reverse calculation solves s = [Ksp/(aᵃbᵇ)]¹⁄⁽ᵃ⁺ᵇ⁾.
Ksp = [M]ᵃ[X]ᵇ = (as)ᵃ(bs)ᵇ- Ksp — Solubility product
- Equilibrium constant for the dissolution expression
- s — Molar solubility
- Moles of formula units dissolved per liter (mol/L)
- a — Cation coefficient
- Cation stoichiometric coefficient
- b — Anion coefficient
- Anion stoichiometric coefficient
One-to-one salt
A 1:1 salt has molar solubility 0.001 mol/L.
- s
- 0.001 mol/L
- a
- 1
- b
- 1
- Ksp = (1 × 0.001)¹(1 × 0.001)¹
Result: Ksp = 1.000000 × 10⁻⁶.
For a 1:1 ideal salt, Ksp equals s squared.
Understanding your results
Stoichiometry changes the exponent
The relationship between Ksp and solubility depends on the balanced dissolution equation.
- A 1:1 salt gives Ksp = s².
- A 1:2 salt gives Ksp = 4s³.
- Ksp itself is temperature dependent.
- Activities replace concentrations in rigorous thermodynamics.
Assumptions
- The entered coefficients match the balanced dissolution equation.
- No common ion, complex formation, acid-base reaction, or competing solid changes the simple model.
- Concentrations approximate activities.
Limitations
- Does not perform full aqueous speciation or precipitation prediction.
- Does not apply activity-coefficient corrections.
- Does not account for temperature changes or multiple equilibria.
Common mistakes
- Ignoring stoichiometric powers.
- Entering ion concentrations instead of molar solubility.
- Using non-integer coefficients from an unbalanced equation.
- Treating the result as temperature independent.
Practical use cases
Equilibrium coursework
Convert between Ksp and molar solubility for simple salts.
Calculation checks
Verify exponent and coefficient arithmetic before advanced equilibrium modeling.
Frequently asked questions
Are Ksp and molar solubility the same?
No. Their relationship depends on the salt’s dissolution stoichiometry.
Does a common ion change solubility?
Yes. This simple calculator does not include an existing common-ion concentration.
Sources and review
- Precipitation and Dissolution — OpenStax Chemistry. Accessed 2026-07-14.
Reviewed 2026-07-14.