Question
Predict whether aqueous solutions of the following salts are acidic, basic, or neutral. Explain using hydrolysis: (a) NaCl, (b) NH₄Cl, (c) CH₃COONa, (d) NH₄CH₃COO (ammonium acetate).
Solution — Step by Step
When a salt dissolves in water, its ions can interact with water molecules. This is called salt hydrolysis. The pH of the resulting solution depends on the relative strengths of the acid and base from which the salt was formed.
Rule of thumb:
- Salt of strong acid + strong base → neutral (pH ≈ 7)
- Salt of strong acid + weak base → acidic (pH < 7)
- Salt of weak acid + strong base → basic (pH > 7)
- Salt of weak acid + weak base → depends on relative vs
Think of it this way: the stronger parent wins. If the acid was stronger, its anion doesn’t react with water (no hydrolysis), but the cation from the weak base does hydrolyse, releasing H⁺.
NaCl is formed from:
- HCl (strong acid): is very large
- NaOH (strong base): is very large
In solution:
- Na⁺ is the conjugate acid of the strong base NaOH → extremely weak acid → does NOT hydrolyse
- Cl⁻ is the conjugate base of the strong acid HCl → extremely weak base → does NOT hydrolyse
Neither ion reacts with water. The solution pH = 7. Neutral.
NH₄Cl is formed from:
- HCl (strong acid)
- NH₃ (weak base):
In solution:
- Cl⁻: conjugate of strong acid → does NOT hydrolyse
- NH₄⁺: conjugate of the weak base NH₃ → IS a weak acid, DOES hydrolyse:
This reaction produces H₃O⁺ (making the solution acidic).
pH < 7. Acidic solution.
The of NH₄⁺ = (of NH₃) = . This is a weak acid (small ), so the solution is mildly acidic.
CH₃COONa is formed from:
- CH₃COOH (weak acid):
- NaOH (strong base)
In solution:
- Na⁺: conjugate of strong base → does NOT hydrolyse
- CH₃COO⁻ (acetate): conjugate of weak acid → IS a weak base, DOES hydrolyse:
This reaction produces OH⁻ (making the solution basic).
pH > 7. Basic solution.
Ammonium acetate is formed from:
- CH₃COOH (weak acid):
- NH₃ (weak base):
BOTH ions hydrolyse:
- NH₄⁺: weak acid hydrolysis → produces H₃O⁺
- CH₃COO⁻: weak base hydrolysis → produces OH⁻
When of the acid = of the base (as here, both ≈ ), the tendencies to produce H⁺ and OH⁻ are equal → the solution is neutral (pH ≈ 7).
General rule for weak acid/weak base salts: pH = 7 if ; acidic if K_a > K_b; basic if K_b > K_a.
For NH₄CH₃COO: since , the solution is neutral, pH ≈ 7.
Why This Works
Salt hydrolysis is the reverse of neutralisation. When a strong acid and weak base react, the neutralisation goes to completion (strong acid wins). The salt formed in water partially reverts this — the weak base’s cation reclaims its proton from water, making the solution acidic.
The key formula connecting acid/base strengths: . This shows that any conjugate pair’s . The weaker the base ( small), the stronger its conjugate acid ( large).
Summary Table
| Salt | Parent Acid | Parent Base | pH |
|---|---|---|---|
| NaCl | HCl (strong) | NaOH (strong) | 7 (neutral) |
| NH₄Cl | HCl (strong) | NH₃ (weak) | < 7 (acidic) |
| CH₃COONa | CH₃COOH (weak) | NaOH (strong) | > 7 (basic) |
| NH₄CH₃COO | CH₃COOH (weak, ) | NH₃ (weak, ) | ≈ 7 (neutral) |
Common Mistake
For NH₄CH₃COO (ammonium acetate), students often assume “weak acid + weak base” means the solution is either strongly acidic or strongly basic. Wrong — both ions hydrolyse, and when their and values are equal, they cancel out perfectly. This salt is famously used as a buffering agent precisely because it maintains near-neutral pH. Always compare and before concluding.
A simple mental model: the ion from the weaker parent does more hydrolysis, and its product (H⁺ or OH⁻) dominates. Strong parent’s ion = spectator (doesn’t hydrolyse). Weak parent’s ion = active hydrolysis. Whichever weak species has a larger K value wins. For NH₄Cl: weak base (NH₃) forms NH₄⁺ which hydrolyses → acidic. For CH₃COONa: weak acid (CH₃COOH) forms CH₃COO⁻ which hydrolyses → basic.