Acid–Base Strength & pH
Year 11 (IGCSE) 🧪 Acids, Bases & Salts Distinguish strong/weak acids; calculate pH = −log[H⁺]; understand Ka.
⚗️ Strong vs Weak Acids
Dissociation
$$\text{Strong (HCl): HCl} \rightarrow \text{H}^+ + \text{Cl}^- \quad (100\%)$$
$$\text{Weak (CH}_3\text{COOH)}: \text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+ \quad (<5\%)$$Strong acids: HCl, H₂SO₄, HNO₃
Weak acids: CH₃COOH, H₂CO₃, H₃PO₄, citric acid
Weak acids: CH₃COOH, H₂CO₃, H₃PO₄, citric acid
🧮 pH Calculations
pH Formula
$$\text{pH} = -\log_{10}[\text{H}^+] \qquad [\text{H}^+] = 10^{-\text{pH}}$$[H⁺] = 0.01 mol/dm³: pH = −log(10⁻²) = 2
pH = 3: [H⁺] = 10⁻³ = 0.001 mol/dm³
pH = 3: [H⁺] = 10⁻³ = 0.001 mol/dm³
Each pH unit = 10× change in [H⁺]. pH 2 is 100× more acidic than pH 4.
🔬 Water Autoionisation
Water Equilibrium
$$K_w = [\text{H}^+][\text{OH}^-] = 10^{-14} \text{ at 25°C}$$ Pure water: [H⁺] = [OH⁻] = 10⁻⁷ mol/dm³ → pH = 7 at 25°C.
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Interactive Demonstration — Acid–Base Strength & pH
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🧮 pH Calculator