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7.12Equilibrium

Common-Ion Effect — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 19 of 19 1 quiz

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Lesson summary

7.12 Common-Ion Effect — AP CHEMISTRY notes: Explain and calculate how a common ion lowers the solubility of a sparingly soluble salt.Common-ion effect — adding a soluble compound that shares an ion with a dissolved salt lowers the salt's solubilityExample — KCl added to saturate…

  • Equilibrium
  • Expression
  • Comparison
  • Molar solubility
  • Example — KCl added to saturated AgCl produces more AgCl(s) because Cl− is the common ion
  • Result — AgCl precipitates and the dissolved Ag+ falls

Part 1 of 6The effect

Common-ion effect
  • Common-ion effect adding a soluble compound that shares an ion with a dissolved salt lowers the salt's solubility
Example
  • KCl added to saturated AgCl produces more AgCl(s) because Cl− is the common ion

Part 2 of 6Explanation using equilibrium

Equilibrium
  • AgCl(s) ⇌ Ag+ + Cl−
Added Cl−
  • Added Cl− shifts the equilibrium to the left (Q > Ksp)
Result
  • AgCl precipitates and the dissolved Ag+ falls

Part 3 of 6Calculation method

1
Write the dissolution equation and Ksp expression.
2
Set up an ICE table where the common ion has a non-zero initial concentration.
3
Assume x is small compared with the initial common-ion concentration.
4
Solve Ksp for x, the molar solubility.

Part 4 of 6Pattern: PbCl₂ in 0.10 M NaCl

Expression
  • Ksp = [Pb2+][Cl−]2 = 1.7 × 10-5
Equilibrium
  • [Pb2+] = x and [Cl−] = 0.10 + 2x ≈ 0.10
Result
  • Result x = 1.7 × 10-5/(0.10)2 = 1.7 × 10-3 M
Comparison
  • Comparison in pure water 4x3 = 1.7 × 10-5 gives x = 0.016 M, so solubility falls about tenfold

Part 5 of 6Precipitation test with a common ion

CaSO4 mixed with Na2SO4 (1.0 × 10-3 M each, equal volumes) — [Ca2+] = 5.0 × 10-4 M and [SO42−] = 1.0 × 10-3 M
Result
  • Q = 5.0 × 10-7 < Ksp = 2.0 × 10-5, so no precipitate forms

Part 6 of 6More from the specification

1Solubility suppression in sparingly soluble salts
  • Solubility suppression in sparingly soluble salts adding common ions to saturated electrolyte solutions precipitates excess solid salts by shifting dissolution equilibrium backward.
2Chloride
  • Salt of hydrochloric acid.
3Sulfate
  • Salt of sulfuric acid.
4Insoluble
  • Unable to dissolve.

Key terms

TermMeaning
Common ionIon shared by two compounds in the same solution.
Common-ion effectDecrease in solubility due to a common ion.
Molar solubilityMoles of salt dissolving per litre.
KspSolubility product, unchanged by a common ion at constant temperature.

Avoid these mistakes

Avoid these mistakes
Ignoring the initial common-ion concentration in the ICE table.
Saying the common ion changes Ksp — it changes solubility only.
Forgetting to halve concentrations when equal volumes are mixed.

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