Hydrolysis of Salts: Acidic, Basic, and Neutral Solutions

Core Concept

When a salt dissolves, its ions may react with water in a process called hydrolysis (literally "water-splitting"). Whether the resulting solution is acidic, basic, or neutral is decided by the ions' parent acid and base. The rule of thumb: the cation of a strong base and the anion of a strong acid are spectators (neutral solution); the cation of a weak base (or a small, highly charged metal ion) makes the solution acidic; and the anion of a weak acid makes it basic.

Key Ideas

• Neutral: strong acid + strong base → neither ion hydrolyzes (NaCl, KNO₃, CaCl₂).
• Acidic: strong acid + weak base → the cation hydrolyzes (NH₄Cl → NH₄⁺ donates H⁺).
• Basic: weak acid + strong base → the anion hydrolyzes (NaCH₃COO → CH₃COO⁻ accepts H⁺).
• Both hydrolyze: weak acid + weak base → compare Ka of the cation to Kb of the anion (NH₄CH₃COO ≈ neutral; NH₄CN is basic).
• Metal-cation acidity: small, highly charged cations (Fe³⁺, Al³⁺, Cu²⁺) polarize water and release H⁺, giving acidic solutions even without a proton to donate.

Equations and Variables

• Acidic cation: NH₄⁺ + H₂O ⇌ H₃O⁺ + NH₃, Ka = Kw/Kb(NH₃)
• Basic anion: CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻, Kb = Kw/Ka(CH₃COOH)
• Metal ion: [Fe(H₂O)₆]³⁺ + H₂O ⇌ [Fe(H₂O)₅OH]²⁺ + H₃O⁺
• For an acidic salt: [H₃O⁺] = √(Ka × C_salt)
• For a basic salt: [OH⁻] = √(Kb × C_salt)

How It Works

1. Split the salt into its ions and identify the "parent" acid and base that would form each ion.
2. The conjugate of a strong acid (e.g., Cl⁻) or strong base (e.g., Na⁺) is too weak to hydrolyze — it is a spectator.
3. The conjugate of a weak acid (CH₃COO⁻) hydrolyzes as a base; the conjugate of a weak base (NH₄⁺) hydrolyzes as an acid.
4. Convert the needed Ka or Kb using Ka × Kb = Kw.
5. Solve with an ICE table to get [H₃O⁺] or [OH⁻], then the pH.
6. When both ions hydrolyze, compare their Ka and Kb to decide the net direction.

Worked Example

Predict whether each solution is acidic, basic, or neutral, and find the pH of 0.10 M NaF (Ka of HF = 6.8 × 10⁻⁴).

• NaCl: Na⁺ (from NaOH, strong) and Cl⁻ (from HCl, strong) → neutral.
• NH₄Cl: Cl⁻ spectator; NH₄⁺ (from weak NH₃) → acidic.
• NaF: Na⁺ spectator; F⁻ is the conjugate base of weak HF → basic.

For 0.10 M NaF:

Kb(F⁻) = Kw / Ka(HF) = (1.0 × 10⁻¹⁴) / (6.8 × 10⁻⁴) = 1.47 × 10⁻¹¹

F⁻ + H₂O ⇌ HF + OH⁻; Kb = x²/(0.10 − x) ≈ x²/0.10

x = [OH⁻] = √(1.47 × 10⁻¹¹ × 0.10) = √(1.47 × 10⁻¹²) = 1.21 × 10⁻⁶ M

pOH = 5.92 → pH = 14.00 − 5.92 = 8.08 (basic) ✓

How it works

1. Split the salt into its ions and identify the "parent" acid and base that would form each ion.
2. The conjugate of a strong acid (e.g., Cl⁻) or strong base (e.g., Na⁺) is too weak to hydrolyze — it is a spectator.
3. The conjugate of a weak acid (CH₃COO⁻) hydrolyzes as a base; the conjugate of a weak base (NH₄⁺) hydrolyzes as an acid.
4. Convert the needed Ka or Kb using Ka × Kb = Kw.
5. Solve with an ICE table to get [H₃O⁺] or [OH⁻], then the pH.
6. When both ions hydrolyze, compare their Ka and Kb to decide the net direction.

Common confusions

• "All salts are neutral." — Only strong-acid/strong-base salts are; many common salts are acidic or basic.
• "Na⁺ or Cl⁻ hydrolyzes." — They are spectators; they are conjugates of a strong base/acid and are too weak to react.
• "NH₄Cl is basic because NH₃ is a base." — NH₄⁺ is the acid form of ammonia; it donates a proton and makes the solution acidic.
• "Metal ions with no H can't be acidic." — Small, highly charged cations (Fe³⁺, Al³⁺) release H⁺ from coordinated water molecules.
• "You can skip the Kw conversion." — For salt pH you must convert the weak parent's Ka to the ion's Kb (or vice versa) with Ka × Kb = Kw.

Quick review

• Neutral: strong acid + strong base.
• Acidic: strong acid + weak base (cation hydrolyzes).
• Basic: weak acid + strong base (anion hydrolyzes).
• Use Ka × Kb = Kw to convert constants for the hydrolyzing ion.
• Metal cations Fe³⁺, Al³⁺, Cu²⁺ → acidic via water polarization.