Solubility Formula

Quantifying dissolution equilibrium and maximum concentration in solution

Solubility Product Constant (Ksp)

General Equilibrium

AₓBᵧ(s) ⇌ xA⁺(aq) + yB⁻(aq)

Ksp = [A⁺]ˣ[B⁻]ʸ

Molar Solubility (s)

Maximum moles that dissolve per liter

s = moles dissolved / L solution

Common Ksp Expressions

AB Type (1:1)

AgCl ⇌ Ag⁺ + Cl⁻

Ksp = [Ag⁺][Cl⁻] = s²

s = √Ksp

ABâ‚‚ Type (1:2)

CaF₂ ⇌ Ca²⁺ + 2F⁻

Ksp = [Ca²⁺][F⁻]² = s(2s)² = 4s³

s = ∛(Ksp/4)

Aâ‚‚B Type (2:1)

Ag₂CrO₄ ⇌ 2Ag⁺ + CrO₄²⁻

Ksp = [Ag⁺]²[CrO₄²⁻] = (2s)²s = 4s³

s = ∛(Ksp/4)

Related Calculators

Solubility Calculator

Calculate Ksp and molar solubility

Solubility Product Calculator

Ksp calculations

Solubility Product Constant

Ksp = [A⁺]^x[B⁻]^y

All Solution Calculators

Explore all solution tools

A₃B Type (3:1)

Ca₃(PO₄)₂ ⇌ 3Ca²⁺ + 2PO₄³⁻

Ksp = [Ca²⁺]³[PO₄³⁻]² = (3s)³(2s)² = 108s⁵

s = ⁵√(Ksp/108)

Worked Examples

Example 1: Simple 1:1 Solubility

Problem: AgCl has Ksp = 1.8 × 10⁻¹⁰. Find molar solubility.

Solution:

AgCl ⇌ Ag⁺ + Cl⁻

Ksp = [Ag⁺][Cl⁻] = s × s = s²

1.8 × 10⁻¹⁰ = s²

s = √(1.8 × 10⁻¹⁰)

s = 1.3 × 10⁻⁵ M

(Very low solubility - AgCl is "insoluble")

Example 2: 1:2 Solubility (CaFâ‚‚)

Problem: CaF₂ has Ksp = 3.9 × 10⁻¹¹. Find molar solubility.

Solution:

CaF₂ ⇌ Ca²⁺ + 2F⁻

If s mol/L dissolves:

[Ca²⁺] = s, [F⁻] = 2s

Ksp = [Ca²⁺][F⁻]² = s(2s)² = 4s³

3.9 × 10⁻¹¹ = 4s³

s³ = 9.75 × 10⁻¹²

s = 2.1 × 10⁻⁴ M

Example 3: Common Ion Effect

Problem: Find solubility of AgCl in 0.10 M NaCl. Ksp(AgCl) = 1.8 × 10⁻¹⁰

Solution:

Initial [Cl⁻] = 0.10 M from NaCl

AgCl ⇌ Ag⁺ + Cl⁻

[Ag⁺] = s, [Cl⁻] = 0.10 + s ≈ 0.10 M (s very small)

Ksp = s(0.10)

1.8 × 10⁻¹⁰ = 0.10s

s = 1.8 × 10⁻⁹ M

140× less soluble than in pure water!

Precipitation Prediction

Reaction Quotient (Q)

Q = [A⁺]ˣ[B⁻]ʸ
✓

Q < Ksp: Unsaturated - no precipitation

âš¡

Q = Ksp: Saturated equilibrium

âš 

Q > Ksp: Supersaturated - precipitation occurs

Common Mistakes

⚠️

Wrong Ksp Expression

For CaF₂: Ksp = s(2s)², NOT s³. Must account for stoichiometric coefficients!

⚠️

Ignoring Common Ion

Solubility decreases in presence of common ion - use initial concentration

💡

Ksp ≠ Solubility

Ksp is equilibrium constant. Solubility (s) is calculated FROM Ksp.

💡

Temperature Dependence

Ksp varies with temperature - values given are typically at 25°C

Related Tools

Solubility Calculator

Calculate from Ksp

Ksp Formula

Detailed Ksp explanation

Solubility Product

Ksp calculations

Common Ion Effect

Solubility in ionic solutions