← Equilibrium principles in aqueous systems

6 pH of salt solutions

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Mnemonic: salt pH

“The weak parent wins the argument.” Whichever parent (acid or base) was weak controls the pH of the salt. Weak acid + strong base → the anion is basic → pH > 7. Strong acid + weak base → the cation is acidic → pH < 7. Both strong → neutral.

Predicting salt pH

  • Salt of strong acid + strong base (e.g. NaCl) → neutral (pH 7).
  • Salt of weak acid + strong base (e.g. CH₃COONa) → basic; the anion is a conjugate base (A⁻ + H₂O ⇌ HA + OH⁻).
  • Salt of strong acid + weak base (e.g. NH₄Cl) → acidic; the cation donates H⁺ (NH₄⁺ ⇌ NH₃ + H⁺).
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Think “which parent was weak”

The ion from the weak parent hydrolyses water and controls the pH. Weak-acid anion → basic; weak-base cation → acidic.

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