Solubility product
For a slightly soluble salt, Ksp is the equilibrium constant for its dissolving. For AgCl(s) ⇌ Ag⁺ + Cl⁻, Ksp = [Ag⁺][Cl⁻]. A smaller Ksp means a less soluble salt (compare only salts with the same ion ratio).
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Analogy
A party room with a fixed capacity: once the limit is reached, extra guests stay outside as solid.
Example
s = [Ag⁺] = [Cl⁻]s² = 1.8×10⁻¹⁰s = 1.3×10⁻⁵ mol/L
In practice
Hard water leaves limescale because calcium carbonate has a small Ksp and precipitates when conditions change.
Practice
Flashcards
- Ksp expression for CaF₂?Ksp = [Ca²⁺][F⁻]².
Interview questions
- Calculate the molar solubility of AgCl (Ksp = 1.8×10⁻¹⁰) in water and in 0.10 M NaCl, and explain the difference.
- In water s = √Ksp = 1.3×10⁻⁵ M
- In NaCl [Cl⁻] = 0.10 M
- s = Ksp/0.10 = 1.8×10⁻⁹ M
- Common ion effect
Questions people ask
How do I tell if a precipitate will form?
Calculate the ion product Q using the concentrations in the mixture, and compare it with Ksp. If Q > Ksp, a precipitate forms. If Q = Ksp, the solution is just saturated. If Q < Ksp, no precipitate forms and more solid can dissolve.
Can I compare Ksp values to rank solubility?
Only for salts that give the same number of ions, such as AgCl and AgBr. For example, AgBr's smaller Ksp means it is less soluble. For salts such as PbCl₂ and AgCl, you must convert Ksp to molar solubility first.