Ionic Equilibrium and Redox
This section covers Ionic Equilibrium and Redox: weak acids and bases, pH, buffers, solubility product, salt hydrolysis, oxidation numbers and balancing redox equations. WBCHSE frequently tests pH calculations and half-reaction balancing.
Core rules
- Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C; pH = −log[H⁺].
- Weak acid: [H⁺] ≈ √(KaC). Buffer: pH = pKa + log([salt]/[acid]).
- Ksp is the product of ion concentrations in a saturated solution; a common ion lowers solubility.
- Oxidation number rules: O is usually −2 but −1 in peroxides; H is +1 except in hydrides. Balance redox by atoms, then charge, using H⁺/OH⁻ and electrons.
Worked example
Calculate the pH of 0.010 M CH₃COOH (Ka = 1.8 × 10⁻⁵). Use [H⁺] = √(KaC) = √(1.8 × 10⁻⁷) = 4.24 × 10⁻⁴ M. pH = −log(4.24 × 10⁻⁴) = 3.37. The approximation is acceptable because dissociation is small. For a buffer made from equal concentrations of acid and salt, pH ≈ pKa = 4.74.
Traps
Do not use √(KaC) when dissociation is large or when a common ion is present; do not call a 10⁻⁸ M HCl solution pH 8; do not forget salt hydrolysis; do not assign oxygen −2 in H₂O₂; balance both atoms and charge in redox equations.