🔬 Ksp Calculator

Calculate Ksp from ion concentrations or molar solubility from Ksp.

Salt Type

Solve for

[Cation] (mol/L)

[Anion] (mol/L)

——

Result

How to Use This Calculator

This tool works in two directions. You can enter measured ion concentrations to calculate Ksp, or enter a known Ksp value to find the molar solubility (s). Start by selecting the salt type, which determines how many ions each formula unit produces and their stoichiometric relationship.

Select the salt type from the dropdown. Choose AB for 1:1 salts like AgCl, AB₂ for salts like CaF₂ that produce one cation and two anions, or A₂B for salts like Ag₂SO₄.

Choose whether to solve for Ksp (enter ion concentrations) or molar solubility s (enter Ksp). Switch the "Solve for" selector accordingly.

Enter either the measured cation and anion concentrations in mol/L, or the Ksp value from a data table. Ksp values are often very small, like 1.8e-10 for AgCl.

Click Calculate. Results are shown in scientific notation to handle the very small numbers typical of sparingly soluble salts.

Ksp Expressions by Salt Type

AB ⇌ A⁺ + B⁻ Ksp = [A⁺][B⁻] = s² AB₂ ⇌ A²⁺ + 2B⁻ Ksp = [A²⁺][B⁻]² = 4s³ A₂B ⇌ 2A⁺ + B²⁻ Ksp = [A⁺]²[B²⁻] = 4s³ AB₃ ⇌ A³⁺ + 3B⁻ Ksp = [A³⁺][B⁻]³ = 27s⁴

s is the molar solubility in mol/L. For a 1:1 salt, Ksp = s², so s = √Ksp. For AB₂ and A₂B types, the coefficient produces the factor of 4 in the expression 4s³. Getting the stoichiometry right is the most common source of errors in these problems.

Worked Examples

AgCl (Ksp = 1.8 × 10⁻¹⁰)s = √(1.8×10⁻¹⁰) = 1.34 × 10⁻⁵ mol/L

CaF₂ (Ksp = 3.9 × 10⁻¹¹)s = (3.9×10⁻¹¹ / 4)^(1/3) = 2.14 × 10⁻⁴ mol/L

Ag₂SO₄ (Ksp = 1.2 × 10⁻⁵)s = (1.2×10⁻⁵ / 4)^(1/3) = 1.44 × 10⁻² mol/L

BaSO₄: [Ba²⁺] = [SO₄²⁻] = 1.05 × 10⁻⁵ mol/LKsp = (1.05×10⁻⁵)² = 1.10 × 10⁻¹⁰

Where This Calculation Comes Up

Ksp calculations appear throughout qualitative analysis, water treatment, and clinical chemistry. In qual analysis, knowing the Ksp of various metal sulfides and hydroxides lets you predict the order in which ions precipitate when you adjust pH. For example, CuS (Ksp = 6×10⁻³⁶) precipitates from acidic solution while MnS (Ksp = 2×10⁻¹³) needs near-neutral conditions. That 23-order-of-magnitude difference in Ksp is exactly what allows selective precipitation to separate Cu²⁺ from Mn²⁺.

In water treatment, Ksp governs whether calcium carbonate will deposit on pipes (scale formation) or stay dissolved. Water that is supersaturated with respect to CaCO₃ (Ksp = 3.3×10⁻⁹) will deposit scale wherever nucleation can occur. The common ion effect is relevant here too: adding Ca²⁺ or CO₃²⁻ from an external source lowers the solubility of CaCO₃ further, shifting the equilibrium toward the solid. You will calculate these exact scenarios in analytical and environmental chemistry courses.

Frequently Asked Questions

What is Ksp?

Ksp (solubility product constant) is the equilibrium constant for a sparingly soluble salt dissolving in water: AB(s) ⇌ A⁺(aq) + B⁻(aq). Ksp = [A⁺][B⁻].

What is molar solubility?

Molar solubility (s) is the moles of salt that dissolve per litre of solution. For AB: Ksp = s². For AB₂: Ksp = 4s³.

When does precipitation occur?

Precipitation occurs when the ion product Q = [A⁺][B⁻] exceeds Ksp. If Q < Ksp, more salt can dissolve.

Example: Ksp of AgCl is 1.8×10⁻¹⁰. What is the molar solubility?

AgCl ⇌ Ag⁺ + Cl⁻. Ksp = s². s = √(1.8×10⁻¹⁰) = 1.34×10⁻⁵ mol/L.

What is the common ion effect?

Adding a common ion (one already present in solution) decreases solubility, shifting equilibrium toward the solid.