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 = $4s^{3}$ . 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?

Initial [Ag⁺] × 10⁻⁵ M1Initial [Cl⁻] × 10⁻⁵ M1

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.

Explore: Use the reversible particle balance to picture dissolving and precipitation, then compare the ion product with Ksp.

AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq) Ksp = [Ag⁺][Cl⁻]

WORKED EXAMPLE

AgCl has Ksp = 1.8 × $10^{-10}$ at the stated temperature. Find its molar solubility in pure water.

Let both ion concentrations equal s.
$s^{2}$ = 1.8 × $10^{-10}$ ; s = 1.34 × $10^{-5}$ mol $L^{-1}$ .

Answer: 1.34e-5 mol $L^{-1}$

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.

QCAA Chemistry 2025 v1.3 · p. 33 ↗

READY TO TRY IT?

When a solid appears.

Will a solid form?

AgCl has Ksp = 1.8 × 10−1010^{-10}10−10 at the stated temperature. Find its molar solubility in pure water.

Turn the idea into an answer.

AgCl has Ksp = 1.8 × $10^{-10}$ at the stated temperature. Find its molar solubility in pure water.