Which of the Following is a Weak Acid? — Definition & Examples

A weak acid partially dissociates in water to produce hydrogen ions (H⁺) and a conjugate base. The dissociation is reversible:

HA(aq) ⇌ H⁺(aq) + A⁻(aq)

Key characteristics: • Low degree of ionisation (less than 5% in most cases) • Small value of acid dissociation constant (Ka) • Establishes equilibrium between dissociated and undissociated forms • Higher pH than strong acids at the same concentration • Acts as a weak electrolyte

Common weak acids include:

1. Acetic acid (CH₃COOH) — vinegar; Ka = 1.8 × 10⁻⁵
2. Carbonic acid (H₂CO₃) — found in fizzy drinks; Ka = 4.3 × 10⁻⁷
3. Hydrofluoric acid (HF) — Ka = 6.8 × 10⁻⁴
4. Citric acid — found in citrus fruits
5. Formic acid (HCOOH) — in ant venom; Ka = 1.8 × 10⁻⁴
6. Nitrous acid (HNO₂) — Ka = 4.5 × 10⁻⁴
7. Phosphoric acid (H₃PO₄) — Ka₁ = 7.5 × 10⁻³
8. Hydrogen sulphide (H₂S) — Ka = 1.0 × 10⁻⁷
9. Hypochlorous acid (HOCl) — Ka = 3.0 × 10⁻⁸
10. Boric acid (H₃BO₃)

Strong acids fully dissociate in water: HCl → H⁺ + Cl⁻ (100% dissociation)

Strong acids: HCl, HNO₃, H₂SO₄, HBr, HI, HClO₄

Weak acids partially dissociate: CH₃COOH ⇌ CH₃COO⁻ + H⁺ (about 1% dissociation)

Key differences: • Degree of ionisation: Strong = 100%; Weak = 1–5% • Ka value: Strong = very large; Weak = very small • Conductivity: Strong acids conduct electricity better • pH: Strong acids have lower pH at same concentration • Reaction rate: Strong acids react faster

To identify whether an acid is weak:

1. Check the Ka value — if Ka is small (< 1), it is a weak acid.
2. Check the degree of ionisation — if less than 100%, it is weak.
3. Common knowledge list — most organic acids are weak.

Memory tip: Only 6 strong acids exist (HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄). All other acids are weak.

In MCQ questions: If options include CH₃COOH (acetic acid), H₂CO₃ (carbonic acid), or HF (hydrofluoric acid) alongside HCl or H₂SO₄, the organic or partially-ionising one is the weak acid.