Year 12 / Ksp and precipitation
When a solid appears.
A sparingly soluble salt establishes an equilibrium between its solid and dissolved ions. The solid is left out of Ksp. For Ag₂CrO₄(s) ⇌ 2Ag⁺ + CrO₄²⁻, Ksp = [Ag⁺]²[CrO₄²⁻].
If the molar solubility in pure water is s, this salt produces [Ag⁺] = 2s and [CrO₄²⁻] = s, so Ksp = . Adding a common ion reduces solubility.
Calculate Qsp with the concentrations after mixing and dilution. Qsp > Ksp predicts precipitation; Qsp < Ksp is unsaturated. Equal values describe saturation.
INTERACTIVE MODEL
Will a solid form?
Qsp = 1.00e-10 · Unsaturated; no precipitate
AgCl: Ksp = 1.8 × 10⁻¹⁰ at the stated temperature. Sliders give the ion concentrations immediately after mixing, before precipitation. Particles are illustrative, not to scale.
AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq) Ksp = [Ag⁺][Cl⁻]
Assumed knowledge
c = n/V; scientific notation; solution volumes are additive in these exercises.
Learning checkpoints & source
YOUR SYLLABUS CHECKPOINT- Write solubility-product expressions.
- Relate molar solubility to ion concentrations.
- Use Qsp to predict precipitation.
READY TO TRY IT?