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Walk-through

How to Use This Calculator

4 steps

1

Pick the salt's dissociation type

Choose AB, AB2, A2B, or AB3 from the Salt Type dropdown, based on how the compound splits into ions when it dissolves. The hint text below the dropdown shows the matching Ksp formula and a few example compounds.

2

Choose a direction

Use the "Ksp from Solubility" tab if you know the molar solubility (mol/L) and want Ksp. Use "Solubility from Ksp" if you already have a Ksp value and want the molar solubility instead.

3

Enter your known value

Type the molar solubility or Ksp value into the single input on the active tab. You can use plain decimals (0.0013) or scientific notation (1.3e-3) — the calculator updates instantly as you type.

4

Read the result

The result card shows the computed Ksp or molar solubility, along with the salt type and formula used. Check the Salt Type Reference tab if you want to confirm which stoichiometry matches your compound.

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Reference

Formula & Methodology

3 formulas

AB salts (1:1 dissociation)

Ksp = s^2

A salt like AgCl dissociates as AB(s) ⇌ A+(aq) + B-(aq), so both ion concentrations equal the molar solubility s. Substituting into Ksp = [A+][B-] gives Ksp = s × s = s^2. Inverting: s = sqrt(Ksp).

AB2 and A2B salts (1:2 or 2:1 dissociation)

Ksp = 4s^3

A salt like Mg(OH)2 dissociates as AB2(s) ⇌ A2+(aq) + 2B-(aq), giving [A2+] = s and [B-] = 2s. Substituting into Ksp = [A2+][B-]^2 gives Ksp = s × (2s)^2 = 4s^3. The same formula applies to A2B salts like Ag2CrO4, where [A+] = 2s and [B2-] = s. Inverting: s = cube-root(Ksp / 4).

AB3 salts (1:3 dissociation)

Ksp = 27s^4

A salt like Fe(OH)3 dissociates as AB3(s) ⇌ A3+(aq) + 3B-(aq), giving [A3+] = s and [B-] = 3s. Substituting into Ksp = [A3+][B-]^3 gives Ksp = s × (3s)^3 = 27s^4. Inverting: s = fourth-root(Ksp / 27).

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Glossary

Key Terms Explained

7 terms

Ksp The solubility product constant — the equilibrium constant for a sparingly soluble ionic solid dissolving into its constituent ions. A smaller Ksp means the salt is less soluble.

Solubility product Another name for Ksp: the product of the equilibrium ion concentrations, each raised to the power of its stoichiometric coefficient, for a saturated solution of a slightly soluble salt.

Molar solubility The maximum concentration (in mol/L) of a salt that can dissolve in water before the solution becomes saturated and a solid precipitate forms. Written as s in Ksp formulas.

Dissociation The process by which an ionic solid separates into its individual cations and anions when it dissolves in water. The stoichiometry of dissociation (1:1, 1:2, 2:1, or 1:3) determines which Ksp formula applies.

Common-ion effect The decrease in a salt's molar solubility when a solution already contains one of the salt's ions from another source (for example, dissolving AgCl in water that already contains Cl- ions). Ksp stays constant, so adding a common ion forces the other ion's equilibrium concentration — and the salt's solubility — down.

Saturated solution A solution holding the maximum amount of dissolved solute possible at a given temperature. In a saturated solution of a sparingly soluble salt, the ion concentrations satisfy Ksp exactly.

Precipitate The solid that forms when the product of a salt's actual ion concentrations exceeds Ksp, pushing the system to remove excess dissolved ions by depositing them back into solid form.

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Scenarios

Real-World Examples

3 worked examples

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Chem 101 student

AB salt: Ksp from solubility

Salt type AB (1:1) Molar solubility, s 1×10⁻³ mol/L

For an AB salt, Ksp = s², so Ksp = (1×10⁻³)² = 1×10⁻⁶. This is the classic textbook setup — a 1:1 salt like AgCl or BaSO₄ where both ions carry the same coefficient, so squaring the solubility gives Ksp directly.

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Lab prep TA

AB2 salt: Ksp from solubility

Salt type AB₂ (1:2) Molar solubility, s 1×10⁻³ mol/L

For an AB₂ salt like Mg(OH)₂, the anion concentration is 2s, so Ksp = s × (2s)² = 4s³ = 4×(1×10⁻³)³ = 4×10⁻⁹. Notice this is much larger than the AB case for the same solubility — the coefficient amplification matters.

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Exam reviewer

Reverse direction: solubility from Ksp

Salt type AB (1:1) Ksp 1.8×10⁻¹⁰

Given Ksp for an AB salt (close to AgCl's real Ksp), invert with s = √Ksp = √(1.8×10⁻¹⁰) ≈ 1.34×10⁻⁵ mol/L. This is the direction used to estimate how much of a slightly soluble salt will actually dissolve in water.

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Deep Dive

Understanding the Ksp Calculator

3 chapters · 4 min read

The solubility product, Ksp, describes how much of a sparingly soluble ionic compound dissolves in water before the solution saturates. This calculator converts between Ksp and molar solubility in either direction, for the four dissociation patterns that cover most general-chemistry salts.

How the Ksp Calculator works

When an ionic solid dissolves, it dissociates into its cation and anion in a fixed ratio set by the compound's formula. At saturation, the solid, undissolved compound is in equilibrium with its dissolved ions, and the equilibrium constant for that process is Ksp — the product of each ion's molar concentration raised to its stoichiometric coefficient.

Because every ion's concentration at saturation is a whole-number multiple of the molar solubility s, substituting those multiples into the Ksp expression collapses to a single formula per stoichiometry: s² for a 1:1 salt, 4s³ for a 1:2 or 2:1 salt, and 27s⁴ for a 1:3 salt. This calculator applies the correct substitution automatically once you pick the salt type.

Inputs and what they mean

Salt type sets which dissociation stoichiometry applies — pick the option matching your compound's formula (for example, Mg(OH)₂ is AB₂ since it produces one Mg²⁺ ion and two OH⁻ ions).

Molar solubility (s), used on the "Ksp from Solubility" tab, is the concentration of dissolved salt in mol/L at saturation. Ksp, used on the "Solubility from Ksp" tab, is a unitless equilibrium constant — enter it in decimal or scientific notation exactly as published in a reference table.

Limits and edge cases

This calculator assumes the salt dissolves in pure water with no other source of its ions present. If a solution already contains a common ion (for example, dissolving AgCl in a solution that already has Cl⁻ from NaCl), the real solubility will be lower than this calculator reports — that's the common-ion effect, and it requires a more detailed equilibrium calculation than a simple Ksp inversion.

The four stoichiometries covered here (AB, AB₂, A₂B, AB₃) handle the large majority of Ksp problems seen in general chemistry, but salts with more complex ratios (such as A₂B₃) need their own custom Ksp expression, which this tool does not compute directly.

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Questions

Frequently Asked Questions

6 questions

What is the Ksp formula, and does it change by salt type?+

Yes — the formula depends on how the salt dissociates. For a 1:1 salt (AB), Ksp = s². For a 1:2 or 2:1 salt (AB2 or A2B), Ksp = 4s³. For a 1:3 salt (AB3), Ksp = 27s⁴. Pick the salt type that matches your compound's dissociation pattern and the calculator applies the right formula.

Why is the AB2 formula 4s³ instead of just s³?+

When AB2 dissociates into A2+ and 2B-, the anion's equilibrium concentration is twice the molar solubility (2s), not just s. Substituting into Ksp = [A2+][B-]^2 gives s × (2s)^2 = 4s^3 — the coefficient 4 comes from squaring that factor of 2, not from anywhere else.

How do I go from Ksp back to molar solubility?+

Use the "Solubility from Ksp" tab and invert the formula for your salt type: s = √Ksp for AB, s = cube-root(Ksp/4) for AB2/A2B, or s = fourth-root(Ksp/27) for AB3. The calculator performs this inversion automatically once you enter Ksp.

What is the common-ion effect, and does this calculator account for it?+

The common-ion effect is the drop in a salt's solubility when the solution already contains one of its ions from another dissolved compound. This calculator assumes the salt dissolves in pure water with no other ion sources, so it does not account for the common-ion effect — the true solubility in a solution with a common ion will be lower than the value shown here.

What units does Ksp use?+

Ksp itself is technically unitless because it is an equilibrium constant expressed in terms of activities, though it's often written with implied mol/L units carried over from concentration. Molar solubility, s, is always expressed in mol/L (moles of dissolved salt per liter of solution).

What does it mean for a solution to be "saturated"?+

A saturated solution contains the maximum amount of dissolved salt possible at a given temperature — any additional salt added will not dissolve and instead remains (or precipitates) as a solid. At saturation, the ion concentrations satisfy the Ksp equation exactly.

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Ksp Calculator

Salt Type

AB (1:1)

AB₂ / A₂B (1:2 or 2:1)

AB₃ (1:3)

Result

How to Use This Calculator

Pick the salt's dissociation type

Choose a direction

Enter your known value

Read the result

Formula & Methodology

Key Terms Explained

Real-World Examples

Chem 101 student

Lab prep TA

Exam reviewer

Understanding the Ksp Calculator

How the Ksp Calculator works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

Ksp Calculator

Salt Type

AB (1:1)

AB₂ / A₂B (1:2 or 2:1)

AB₃ (1:3)

Result

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How to Use This Calculator

Pick the salt's dissociation type

Choose a direction

Enter your known value

Read the result

Formula & Methodology

Key Terms Explained

Real-World Examples

Understanding the Ksp Calculator

How the Ksp Calculator works

Inputs and what they mean

Limits and edge cases

Frequently Asked Questions

Get a branded PDF of your results

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