Strong vs Weak Acids
Strong acid
A strong acid ionizes completely in water: essentially every acid molecule donates its proton, leaving no intact acid molecules in solution. Its acid dissociation constant Ka is very large, and the pH of its solution can be found directly from its concentration.
Example: Hydrochloric acid, HCl: 0.10 M HCl gives [H+] = 0.10 M, so pH = 1.00 — every HCl molecule contributes a proton.
Weak acid
A weak acid ionizes only partially in water, establishing a dynamic equilibrium between intact acid molecules and the ions they produce. Its Ka is small, so the hydrogen-ion concentration is much lower than the acid's nominal concentration, and pH calculation needs the equilibrium expression.
Example: Acetic acid, CH3COOH (vinegar): 0.10 M gives pH ≈ 2.9, because only about 1.3% of the molecules donate protons.
Key differences
| Aspect | Strong acid | Weak acid |
|---|---|---|
| Dissociation in water | Ionizes essentially 100% — every acid molecule donates its proton. | Ionizes only partially (often 1–5%) — most molecules stay intact, in equilibrium with the ions. |
| Acid dissociation constant Ka | Very large (Ka >> 1), often not even tabulated because ionization is complete. | Small (Ka << 1); for acetic acid, Ka = 1.8 × 10^-5 at 25 °C. |
| pH calculation | [H+] equals the acid's initial concentration, so pH = -log(c) directly. | [H+] is much lower than the initial concentration; requires the Ka expression or an ICE-table calculation. |
| Common examples | HCl, HBr, HI, HNO3, H2SO4 (first proton), HClO4. | CH3COOH (acetic/vinegar acid), HF, H2CO3 (carbonic), HCN, citric acid. |
| Conjugate base | The conjugate base is extremely weak — essentially a spectator that will not re-capture protons. | The conjugate base is relatively strong and can take protons back, which is why weak acids make useful buffers. |
Strength is not the same as concentration — a strong acid can be dilute and harmless-ish, while a weak acid can be concentrated and corrosive. Students also assume 'weak acid' means 'safe': concentrated acetic acid and hydrofluoric acid are both weak acids but can cause serious burns. Finally, pH depends on both strength AND concentration, so you must know both to calculate it.