What is the solubility product constant (Ksp)?

Ksp is the equilibrium constant for the dissolution of a sparingly soluble ionic solid. For a salt MₓAᵧ → x M + y A, Ksp = [M]^x · [A]^y at equilibrium. Pure solids are excluded from the expression because their activity is 1.

How do I convert molar solubility (s) to Ksp?

Express each ion concentration in terms of s, then substitute into the Ksp expression. For AB: [A]=[B]=s → Ksp=s². For AB₂: [A]=s, [B]=2s → Ksp=4s³. For A₂B: [A]=2s, [B]=s → Ksp=4s³. For AB₃: Ksp=27s⁴. This calculator performs that substitution automatically.

What is the reaction quotient Q and how does it predict precipitation?

Q is calculated the same way as Ksp but uses actual (non-equilibrium) concentrations. If Q > Ksp, the solution is supersaturated and a precipitate will form. If Q < Ksp, the solution is unsaturated and more solid can dissolve. If Q = Ksp, the system is at equilibrium.

Does Ksp change with temperature?

Yes. Ksp is temperature-dependent. Most salts become more soluble at higher temperatures (endothermic dissolution), so Ksp increases with temperature. The values in this reference table are for 25°C (298 K) under standard conditions.

Why does the common ion effect reduce solubility?

Adding a salt that shares an ion with the sparingly soluble compound shifts the dissolution equilibrium toward the solid (Le Chatelier's principle), reducing molar solubility. For example, AgCl dissolves far less in NaCl solution than in pure water because [Cl⁻] is already high.

Solubility Product Calculator

Look up or calculate Ksp and molar solubility for sparingly soluble salts — includes precipitate prediction via the reaction quotient Q.

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Salt formula type

Molar solubility s (mol/L)

Salt	Formula	Ksp (25°C)

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Summary

Look up or calculate Ksp and molar solubility for sparingly soluble salts — includes precipitate prediction via the reaction quotient Q.

How it works

Select a calculation mode: Ksp from solubility, solubility from Ksp, or precipitate prediction.
Choose a preset salt from the reference table or enter a custom formula type and Ksp.
Enter the required numeric values (molar solubility, Ksp, or ion concentrations).
Click Calculate to see the result with the equilibrium expression and step-by-step work.
Use the reference table tab to browse Ksp values for 40+ sparingly soluble salts.

Use cases

Determine whether two ionic solutions will form a precipitate when mixed.
Convert a measured molar solubility into a Ksp value for a lab report.
Look up Ksp values for common salts in a general or analytical chemistry course.
Predict the solubility of a salt in pure water versus a common-ion solution.
Verify precipitation conditions in water treatment and environmental chemistry.
Solve equilibrium problems involving AgCl, BaSO4, CaF2, and similar salts.

Frequently Asked Questions

Last updated: 2026-09-22 · Reviewed by Nham Vu

Solubility Product Calculator

Use the Solubility Product Calculator

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