Acid–base equilibria
This domain covers pH of strong and weak species, hydrolysis of salts, buffer design and the shape of titration curves. Marks come from stating your approximations explicitly - for example neglecting water autoionisation - and reporting pH to a sensible number of decimals.
Strong and weak acids and bases
Strong acids and bases dissociate completely, so pH = −log[H₃O⁺] directly from the concentration, remembering Ba(OH)₂ releases two OH⁻ per formula unit. For weak acids, Ka·Kb = Kw = 1.0 × 10⁻¹⁴ at 25 °C, and pH = ½(pKa − log c) when dissociation is under about 5%. For a conjugate pair, pKa + pKb = 14.
Hydrolysis of salts
A salt of a strong acid and strong base is neutral. A salt of a weak acid (NaCH₃COO) gives a basic solution because the anion hydrolyses with Kb = Kw/Ka. A salt of a weak base (NH₄Cl) gives an acidic solution with Ka = Kw/Kb. When both ions hydrolyse, compare Ka and Kb to decide.
Buffers
A buffer is a weak acid with its conjugate base (or the reverse). Henderson–Hasselbalch: pH = pKa + log([base]/[acid]). Added H⁺ is consumed by the base component, so pH barely moves. Capacity is greatest near [base]/[acid] = 1, and the useful range is pKa ± 1.
Titration curves
Strong acid–strong base: equivalence at pH 7, steep jump, phenolphthalein or methyl orange both work. Weak acid–strong base: half-equivalence pH = pKa, a buffer region precedes equivalence, and equivalence pH > 7, so phenolphthalein is correct. Weak base–strong acid: equivalence pH < 7, so use methyl orange.
Worked example
Find the pH of 0.10 M acetic acid with Ka = 1.8 × 10⁻⁵. Let x = [H₃O⁺]; Ka = x²/(0.10 − x) ≈ x²/0.10, so x = √(1.8 × 10⁻⁶) = 1.34 × 10⁻³ M. The dissociation is 1.34%, so the approximation is valid. pH = −log(1.34 × 10⁻³) = 2.87 and the degree of dissociation is 0.0134.
Traps
- Reporting pH with three or four decimals, or attaching units to it.
- Substituting the initial concentration for the equilibrium one without checking the 5% rule.
- Picking an indicator whose pKa is far from the equivalence pH.
- Forgetting that diluting a weak acid raises its degree of dissociation while lowering [H₃O⁺].