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8.11Acids and Bases

pH and Solubility — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 9 parts Lesson 16 of 16 1 quiz

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Lesson summary

8.11 pH and Solubility — AP CHEMISTRY notes: Explain how pH changes the solubility of salts whose ions react with H₃O⁺ or OH⁻.Le Châtelier's principle — changing pH changes the concentration of an ion in the Ksp equilibrium and shifts the dissolutionIons that react — basic anions…

  • Acidic cation
  • Common-ion effect
  • Molar solubility
  • Lower pH — H3O+ removes OH−, shifting dissolution forward, so solubility increases
  • Higher pH — added OH− is a common ion, shifting dissolution backward, so solubility decreases
  • Lower pH — F− reacts with H3O+, shifting dissolution forward, so solubility increases

Section 1 of 9Principle

Le Châtelier's principle
  • Le Châtelier's principle changing pH changes the concentration of an ion in the Ksp equilibrium and shifts the dissolution
Ions that react
  • Ions that react basic anions react with H3O+; acidic cations react with OH−
    • (conjugate bases of weak acids)

Section 2 of 9Which ions react

Ion typeReactionExamples
Anion of a weak acidActs as a base, reacts with H3O+F−, C2O42−, OH−
Anion of a strong acidNegligibleNO3−, Cl−
Group 1 and 2 cationsNegligibleNa+, Ba2+
Highly charged metal cationsAcidic, react with OH−Al3+

Section 3 of 9Fe(OH)₃

Lower pH
  • H3O+ removes OH−, shifting dissolution forward, so solubility increases
Higher pH
  • Higher pH added OH− is a common ion, shifting dissolution backward, so solubility decreases

Section 4 of 9BaF₂

Lower pH
  • F− reacts with H3O+, shifting dissolution forward, so solubility increases
Higher pH
  • Ba2+ is negligible, so solubility is unaffected

Section 5 of 9Al(NO₃)₃

Lower pH
  • NO3− does not react, so solubility is unaffected
Higher pH
  • Al3+ reacts with OH−, shifting dissolution forward, so solubility increases

Section 6 of 9PbC₂O₄ with strong acid

1Two steps
  • C2O42− + H3O+ → HC2O4− and then H2C2O4
2Effect
  • Effect the anion concentration falls, so the equilibrium shifts forward and solubility increases

Section 7 of 9More from the specification

More from the specification
Property
  • A feature that describes what a substance is like and what it does.
Buffer capacity
  • How much added acid or base a buffer can absorb before its pH changes significantly.
Henderson-Hasselbalch equation
  • PH = pKa + log([A−]/[HA]).
Equilibrium position
  • Point of minimum free energy.
Buffer
  • A solution that resists change in pH on adding small amounts of acid or alkali.
Ethanoate
  • The salt of ethanoic acid.
Hydrogen
  • A gas that gives a squeaky pop with a lighted splint.

Section 8 of 9Key terms

TermMeaning
Basic anionAnion that is the conjugate base of a weak acid.
Acidic cationCation that reacts with OH− or hydrolyses water.
Common-ion effectLower solubility due to a shared ion.
Molar solubilityMoles dissolving per litre.

Section 9 of 9Exam technique

Avoid these mistakes
Saying all salts are more soluble in acid — only those with basic anions.
Ignoring the common-ion effect of OH− on hydroxides.
Treating Cl− and NO3− as basic anions.

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