IB Chemistry: Strong vs Weak Acids (Ka, pKa) — HL Guide

A strong acid dissociates almost completely in water; a weak acid dissociates only partially, existing in equilibrium with its conjugate base. IB Chemistry examines this through Ka, pKa, pH calculations, and buffer questions — mostly under Reactions (structure 8.3) at HL, with calculation-heavy Paper 2 and Paper 3 questions.

Ka is the acid dissociation constant, measuring how far a weak acid ionises in water. For HA ⇌ H⁺ + A⁻, Ka = [H⁺][A⁻]/[HA]. A larger Ka means more dissociation and a stronger weak acid; strong acids have such enormous Ka values they're treated as 100% dissociated.

pKa is -log₁₀(Ka), which turns an awkward, tiny number into a manageable scale — exactly like pH does for [H⁺]. A lower pKa means a stronger acid. HL students must convert fluently between the two: pKa = -log Ka, and Ka = 10⁻ᵖᴷᵃ.

Use the Ka expression, assuming [H⁺] ≈ [A⁻] and that dissociation is small enough that [HA] stays close to its starting concentration. Solve Ka = x²/[HA] for x = [H⁺], then take pH = -log[H⁺]. This ICE-table shortcut is exactly what examiners expect on Paper 2.

The Henderson-Hasselbalch equation, pH = pKa + log([A⁻]/[HA]), lets you find a buffer's pH directly from the ratio of conjugate base to acid, skipping the full ICE table. This is HL-only content under structure 8.3, and it comes up regularly in Paper 2 buffer questions.

No — the IB data booklet lists the Ka and pKa values you'll need. Memorising them isn't required. What you do need solid is the Ka expression itself, the pKa-Ka conversion, and the ICE-table method that turns a booklet value into an actual pH.

Expect 'calculate' for numeric Ka, pH or pKa problems, 'deduce' when you must infer relative acid strength from data given, 'compare' for strong-versus-weak behaviour, and 'explain' for the equilibrium reasoning behind partial dissociation. These come straight from the IB's command term glossary, and examiners mark strictly against them.

It's one of the more calculation-heavy corners of HL Chemistry, but not the conceptually hardest topic. Most students who struggle here are shaky on equilibrium algebra, not the underlying chemistry. Once ICE tables from structure 7.1 (equilibrium) feel routine, the acid-base maths follows the exact same pattern.

Strong/weak describes how completely an acid ionises — a fixed chemical property. Concentrated/dilute describes how much acid you dissolved in water — a matter of quantity. You can have a dilute strong acid or a concentrated weak acid; the two ideas are independent, and examiners regularly test whether students confuse them.

A titration curve's shape, and where its equivalence point sits, depends on whether you're titrating a strong or weak acid — a weak acid's starting pH and buffering region look very different from a strong acid's. HL students use this, alongside pKa ranges, to justify indicator choice under structure 8.3.

Ka, pKa and buffer calculations sit within Reactions (structure 8.3), one slice of five HL structure areas, so it's meaningful but not dominant across Paper 2 and Paper 3. Marks here usually come down to steady equilibrium-algebra practice rather than raw memorisation — the kind of skill built through repeated worked problems over the two-year course.

Work backwards from the equilibrium topic first, since weak acid calculations are just ICE tables in disguise. Then drill Ka-to-pKa conversions, pH calculations, and Henderson-Hasselbalch until they're automatic, using timed Paper 2-style questions rather than re-reading notes. Past-paper repetition beats highlighting every time for this topic.