Ksp Solubility Product Calculator

Calculate molar solubility, ion concentrations, and common ion effect using Ksp.

AgCl: 1.8e-10, BaSO4: 1.1e-10, CaCO3: 3.4e-9
For common ion effect
For precipitation check
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Why Ksp Values Cannot Be Compared Directly

Ksp is the equilibrium constant for a solid dissolving into its ions. The trap is that the relationship between Ksp and actual solubility depends on the salt's stoichiometry, so a smaller Ksp does not always mean a less soluble salt.

Salt typeExpressionSolubility s
AB (AgCl)Ksp = s²√Ksp
AB2 (CaF2)Ksp = 4s³(Ksp/4)1/3
AB3 (Fe(OH)3)Ksp = 27s4(Ksp/27)1/4
A2B3Ksp = 108s5(Ksp/108)1/5

The coefficients come from the stoichiometry: for CaF2, [Ca2+] = s but [F−] = 2s, so Ksp = s(2s)² = 4s³. Comparing an AB salt with an AB2 salt by Ksp alone is meaningless — convert both to solubility first.

The Common Ion Effect

Adding a shared ion shifts the equilibrium back toward the solid, suppressing solubility dramatically. AgCl dissolves at 1.34 × 10−5 M in pure water, but in 0.1 M NaCl the chloride is already supplied, so silver drops to 1.8 × 10−9 M — a 7,400-fold reduction.

This is exploited routinely: washing a precipitate with dilute solution of a common ion rather than pure water minimises losses during filtration.

Q vs KspMeaningResult
Q < KspUnsaturatedMore solid dissolves
Q = KspSaturatedEquilibrium
Q > KspSupersaturatedPrecipitate forms

Worked Examples

Example 1: AgCl solubility: Ksp=1.8e-10, 1:1 salt
s=√(1.8e-10)
Result: s=1.34×10⁻⁵ mol/L = 1.9 mg/L
Sparingly soluble — good gravimetric analysis
Example 2: AgCl with 0.1M NaCl (common ion)
s=Ksp/[Cl-]=1.8e-10/0.1
Result: s=1.8×10⁻⁹ mol/L — 10,000× reduction!
Common ion effect: basis of gravimetric precipitation
Example 3: Why stoichiometry matters
AgCl Ksp = 1.8×10−10 versus CaF2 Ksp = 3.9×10−11
Result: CaF2 is more soluble
Smaller Ksp but a cube root instead of a square root: s = 2.1×10−4 M for CaF2 against 1.34×10−5 for AgCl.
Example 4: pH and hydroxide solubility
Mg(OH)2 in neutral versus acidic solution
Result: Far more soluble in acid
Acid consumes hydroxide, lowering [OH−] and pulling the dissolution equilibrium forward. This is why antacids work.

Common Mistakes

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Comparing Ksp values across different salt types

A 1:1 and a 1:2 salt relate to solubility through different powers. Convert both to molar solubility before comparing.

⚠️
Forgetting the stoichiometric coefficient

For CaF2, fluoride concentration is 2s, and it is squared in the expression. This gives Ksp = 4s³, not s³.

⚠️
Ignoring pH for salts of weak acids

Carbonates, sulfides and hydroxides dissolve far more in acid, because protonation removes the anion and pulls the equilibrium forward.

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Assuming precipitation happens the instant Q exceeds Ksp

Supersaturated solutions can persist without nucleation sites. Precipitation may require seeding or time.

Frequently Asked Questions

Common ion effect applications?
Gravimetric analysis: excess precipitating agent ensures quantitative precipitation. Qualitative analysis: H2S in acid (H+ suppresses S²⁻) vs base (higher S²⁻) separates metal sulfides. Pharmaceutical solubility: salt selection affects dissolution.
Ksp and temperature?
Most salts: Ksp increases with T (endothermic dissolution). Exception: Ce2(SO4)3, Na2SO4×10H2O: Ksp decreases with T. Sparingly soluble salts: Ksp often measured at 25°C standard. Temperature change can shift precipitation equilibria in industrial crystallization.
Does a smaller Ksp always mean lower solubility?
No — only within the same stoichiometric class. CaF2 has a smaller Ksp than AgCl yet is more soluble, because the relationship involves a cube root rather than a square root.
What is the common ion effect?
Adding an ion already present in the equilibrium shifts it back toward the solid. AgCl solubility falls thousands of times in 0.1 M NaCl.
Why does pH affect some solubilities?
For salts of weak acids, protonating the anion removes it from solution and pulls the dissolution equilibrium forward. Carbonates and hydroxides are strongly pH dependent.
How do I predict whether a precipitate forms?
Compare the ion product Q with Ksp. Precipitation is expected when Q exceeds Ksp, though supersaturation can delay it.

Formula Explorer connections

Interpretation: This formula describes how reactants, products, ions or phases distribute when opposing processes reach equilibrium. Assumption: Use equilibrium rather than initial concentrations, correct stoichiometric exponents, and the specified temperature; activities may replace concentrations in nonideal systems.

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