pH of Salt Solutions HL Only

Full Notes

Salt Hydrolysis

When a salt dissolves in water, its ions may interact with water molecules in a process called hydrolysis.

Whether salt a solution is acidic, basic or neutral depends on the strengths of the acid and base from which the salt is formed and how its ions interact with water.

Strong Acid + Strong Base → Neutral Salt

Example Sodium chloride (NaCl)

Neither Na⁺ nor Cl⁻ hydrolyzes water.
pH ≈ 7

Strong Acid + Weak Base → Acidic Salt

Example Ammonium chloride (NH₄Cl)

NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺
The ammonium ion hydrolyzes water, producing H₃O⁺ and lowering the pH.
pH < 7

Weak Acid + Strong Base → Basic Salt

Example Sodium ethanoate (CH₃COONa)

CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻
The ethanoate ion hydrolyzes water, producing OH⁻ and raising the pH.
pH > 7

Weak Acid + Weak Base → Variable Salt

The relative strengths of the acid and base (K_a and K_b values) determine whether the salt is acidic, basic, or neutral.

Common Ions and Their pH Effects

NH₄⁺ (ammonium ion): Acts as a weak acid.
NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺
Forms acidic solution.

RCOO⁻ (carboxylate ion): Conjugate base of a weak acid.
RCOO⁻ + H₂O ⇌ RCOOH + OH⁻
Forms alkaline solution.

CO₃²⁻ (carbonate ion):
CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻
Forms alkaline solution.

HCO₃⁻ (hydrogencarbonate ion): Amphiprotic; can act as acid or base.

• As an acid: HCO₃⁻ + H₂O ⇌ CO₃²⁻ + H₃O⁺
• As a base: HCO₃⁻ + H₂O ⇌ H₂CO₃ + OH⁻

Summary

• Salt solutions may be acidic, basic, or neutral depending on parent acid and base strength.
• Neutral salts form from strong acids and bases.
• Acidic salts form from strong acids and weak bases.
• Basic salts form from weak acids and strong bases.
• Hydrolysis of ions such as NH₄⁺, RCOO⁻, CO₃²⁻, and HCO₃⁻ alters solution pH.