Chemistry · General chemistry II · Worked example
Predict whether a precipitate forms
50.0 mL of 1.0 × 10⁻³ M AgNO₃ is mixed with 50.0 mL of 1.0 × 10⁻³ M NaCl. Does AgCl (Ksp = 1.8 × 10⁻¹⁰) precipitate?
Dilute on mixing
The total volume is 100.0 mL, so each ion is diluted to half its stock concentration: [Ag⁺] = [Cl⁻] = 5.0 × 10⁻⁴ M.
Compute Q
Q has the form of Ksp but uses the concentrations just after mixing.
Compare with Ksp
Q = 2.5 × 10⁻⁷ is far larger than Ksp = 1.8 × 10⁻¹⁰: the solution is supersaturated, and AgCl precipitates until Q falls to Ksp. Nearly all the silver leaves the solution; about 1.3 × 10⁻⁵ M of each ion remains.
Result
Yes. Q = 2.5 × 10⁻⁷ > Ksp = 1.8 × 10⁻¹⁰, so silver chloride precipitates.
Your turn
10.0 mL of 0.0020 M CaCl₂ is mixed with 10.0 mL of 0.0020 M NaF. Does CaF₂ (Ksp = 3.7 × 10⁻¹¹) precipitate?
Show the answer and explanation
Yes.
After mixing, [Ca²⁺] = [F⁻] = 0.0010 M, so Q = [Ca²⁺][F⁻]² = (0.0010)(0.0010)² = 1.0 × 10⁻⁹, which is greater than Ksp.
Keep exploring
In Solubility & precipitation, lower both stock solutions to 1.0 × 10⁻⁵ M. Now Q = 2.5 × 10⁻¹¹ is below Ksp, and nothing precipitates.
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