# Solubility Product (Ksp) Calculator

Calculate Ksp from molar solubility or find molar solubility from Ksp. Supports AB, A2B, AB2, A3B salt types and the common ion effect.

## What this calculates

Calculate the solubility product constant (Ksp) from molar solubility, or find how much of a salt dissolves given its Ksp. Supports common salt types (AB, A2B, AB2, A3B) and the common ion effect, which reduces solubility when a shared ion is already present in solution.

## Inputs

- **Solve For** — options: Molar Solubility from Ksp, Ksp from Molar Solubility — Choose whether to find molar solubility or Ksp.
- **Salt Type** — options: AB (e.g., AgCl, BaSO4), A2B (e.g., Ag2CrO4), AB2 (e.g., CaF2, PbCl2), A3B (e.g., Ag3PO4) — Select the stoichiometric type of the salt. This determines the Ksp expression.
- **Ksp Value** — min 0 — The solubility product constant. Use scientific notation for very small values.
- **Molar Solubility (s)** (mol/L) — min 0 — Molar solubility in mol/L. Used when calculating Ksp.
- **Common Ion Concentration** (mol/L) — min 0 — Concentration of a common ion already in solution. Leave at 0 for pure water.
- **Common Ion Is** — options: Cation (A), Anion (B) — Is the common ion the cation or anion of the salt?

## Outputs

- **Result** — formatted as text — The calculated Ksp or molar solubility.
- **Ksp Expression** — formatted as text — The equilibrium expression for the salt.
- **Calculation Steps** — formatted as text — Step-by-step solution.

## Details

The solubility product constant (Ksp) describes how much of a sparingly soluble salt dissolves in water. It is the equilibrium constant for the dissolution reaction.

**What Is Ksp?**

For a generic salt AxBy that dissolves as:

AxBy(s) -> xA(aq) + yB(aq)

The solubility product is:

Ksp = [A]^x * [B]^y

**Common Salt Types**

| Type | Example | Ksp Expression |
|------|---------|---------------|
| AB | AgCl | Ksp = s^2 |
| A2B | Ag2CrO4 | Ksp = (2s)^2(s) = 4s^3 |
| AB2 | CaF2 | Ksp = (s)(2s)^2 = 4s^3 |
| A3B | Ag3PO4 | Ksp = (3s)^3(s) = 27s^4 |

Where s = molar solubility.

**Worked Example: AgCl**

AgCl has Ksp = 1.77 x 10^-10.

s = sqrt(Ksp) = sqrt(1.77 x 10^-10) = 1.33 x 10^-5 mol/L

That means only 0.00133% of a mole of AgCl dissolves per liter of water.

**The Common Ion Effect**

If you dissolve AgCl in a 0.10 M NaCl solution, the Cl- already present pushes the equilibrium back:

Ksp = [Ag+][Cl-] = s(0.10 + s) ≈ s(0.10)

s = 1.77 x 10^-10 / 0.10 = 1.77 x 10^-9 mol/L

The solubility drops by nearly 10,000x compared to pure water. This is the common ion effect in action.

## Frequently Asked Questions

**Q: What is a solubility product constant?**

A: Ksp is the equilibrium constant for the dissolution of a sparingly soluble ionic compound in water. It equals the product of the ion concentrations, each raised to the power of its stoichiometric coefficient. A larger Ksp means the salt is more soluble.

**Q: How does the common ion effect work?**

A: The common ion effect reduces the solubility of a salt when one of its ions is already present in solution. By Le Chatelier's principle, adding a common ion shifts the dissolution equilibrium backward, so less salt dissolves. For example, AgCl is far less soluble in NaCl solution than in pure water.

**Q: Can I compare Ksp values of different salt types?**

A: You can only directly compare Ksp values for salts with the same stoichiometry (the same type of Ksp expression). Comparing Ksp of AgCl (AB type, Ksp = s^2) to CaF2 (AB2 type, Ksp = 4s^3) is misleading because the expressions have different powers. Convert to molar solubility first for a fair comparison.

**Q: What is the difference between Ksp and solubility?**

A: Ksp is a constant at a given temperature that describes the equilibrium between the solid and its ions. Solubility is the actual amount that dissolves, which can change depending on what else is in solution (common ion effect, pH, complex formation). Ksp stays the same; solubility does not.

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Source: https://vastcalc.com/calculators/chemistry/solubility-product
Category: Chemistry
Last updated: 2026-04-08