Ksp Calculator
Find molar solubility from Ksp for 1:1, 2:1, 1:2, 3:1, and 1:3 salts.
What it does
The solubility product constant summarizes how much of a sparingly soluble salt dissolves in water at equilibrium. Big Ksp means an eager dissolver like AgNO3; tiny Ksp means a stubborn one like AgCl.
Turning Ksp into an actual concentration is where stoichiometry bites. Every dissolved formula unit of CaF2 releases one calcium ion and two fluoride ions, so the equilibrium expression carries powers and a divisor that plain square roots miss.
The general form covers every ratio: for a salt AxBy, molar solubility s equals the Ksp divided by a^a times b^b, all raised to 1/(a+b). For 1:1 salts that reduces to the square root, and for 1:2 salts the Ksp divides by 4 before the cube root.
The tool reports molar solubility plus the equilibrium concentration of each ion, because that split is what later calculations need, from common ion problems to whether a precipitate forms when two solutions mix.
The model assumes pure water at a fixed temperature with no activity corrections and no side reactions. Hydroxide salts shift with pH, and complexing agents pull metal ions away, so treat the output as the textbook baseline.
How to use
- Look up or measure the Ksp for your salt.
- Pick the formula type from the ratios.
- Enter Ksp in scientific notation like 6.3e-9.
- Read molar solubility in moles per liter.
- Read the metal and anion concentrations.
- Compare with a table value as a sanity check.
Formula
- Ksp
- Solubility product constant at the working temperature.
- a
- Metal ions per formula unit, the first number in the ratio.
- b
- Anions per formula unit, the second number.
- s
- Molar solubility, moles of formula units dissolved per liter.
The divisor follows the dissociation: 1:1 salts divide by 1 with a square root, 2:1 and 1:2 salts divide by 4 with a cube root, and 3:1 divides by 27 with a fourth root. Ions themselves report as a x s and b x s, so a 2:1 salt shows a metal concentration twice its own solubility. Ksp values themselves come from measured concentrations, and only the salt type is assumed here.
Worked example
Example: CuBr with Ksp 6.3e-9
Copper(I) bromide, a 1:1 salt, with Ksp 6.3e-9 (defaults).
- Type 1:1, so the divisor is 1 x 1 = 1 and the root is a square root.
- s = (6.3e-9 / 1)^(1/2) = 7.94e-5.
- Rounded: 7.9e-5 M.
- Both ions equal s for a 1:1 salt: 7.9e-5 M each.
- Tool shows 7.9e-5 M for solubility, metal, and anion.
Reference table
| Salt | Ksp | Type | Molar solubility |
|---|---|---|---|
| AgCl | 1.77e-10 | 1:1 | 1.3e-5 M |
| CuBr | 6.3e-9 | 1:1 | 7.9e-5 M |
| Ag2CrO4 | 2.76e-12 | 2:1 | 8.8e-5 M |
| CaF2 | 3.45e-11 | 1:2 | 2.1e-4 M |
| Ca(OH)2 | 5.5e-6 | 1:2 | 1.1e-2 M |
Limits
- Pure water only; a common ion changes everything.
- No activity corrections, so concentrated results drift.
- Ksp itself moves with temperature.
- Ignores complexes and acid base side reactions, which matter for hydroxides.
Common mistakes
- Reading Ksp as the solubility itself instead of a product of powers.
- Taking a square root of a 1:2 salt and skipping the divisor of 4.
- Swapping 2:1 and 1:2: solubility matches, ion split does not.
- Applying the result to seawater or brine where a common ion dominates.
- Rounding hard before the root and losing a significant figure.
Try it yourself
Quick checks. Answers sit tucked under each question.
CaF2, Ksp 3.45e-11, 1:2?
2.1e-4 M solubility; metal ion 2.1e-4 M, anion 4.1e-4 M.
Ag2CrO4, Ksp 2.76e-12, 2:1?
8.8e-5 M solubility; metal ion 1.8e-4 M, anion 8.8e-5 M.
Ca(OH)2, Ksp 5.5e-6, 1:2?
1.1e-2 M solubility; metal ion 1.1e-2 M, anion 2.2e-2 M.
Questions
What is Ksp?
The equilibrium constant for dissolving a sparingly soluble salt, written as the product of ion concentrations each raised to its stoichiometric coefficient.
Why divide Ksp by 4 for CaF2?
Dissolving one CaF2 unit gives one Ca2+ and two F-, so Ksp equals 4s^3 and s equals the cube root of Ksp over 4.
Which is more soluble, AgCl or Ag2CrO4?
Ag2CrO4 by molar solubility: 8.8e-5 M versus 1.3e-5 M. Comparing raw Ksp values across different salt types leads to the wrong answer.
Does pH matter?
For hydroxides, hugely. Lower pH consumes OH-, pulls the equilibrium toward dissolving, and adds solubility the simple model misses.
What are the units?
Molar solubility lands in mol/L. Ksp carries units raised to its formula's power, which chemists usually leave unitless for comparison.
Does temperature change the answer?
Yes. Ksp is temperature specific, so values are quoted near 25 C and shift for hot or cold solutions.
Sources
- LibreTexts Chemistry 17.2: molar solubility and Ksp
- OpenStax Chemistry 2e 15.1: precipitation and dissolution