IB Chemistry: Acids, Bases & the pH Scale — FAQs

SL covers Brønsted-Lowry theory, pH/pOH calculations, and strong versus weak acids qualitatively. HL adds Ka, Kb, pKa, pKb calculations, buffer solutions, pH titration curves and the Lewis acid-base model. According to the IB Chemistry guide (first exams 2025), both sit under Reactivity 3.1, with HL content layered on as extensions, not a separate strand.

A strong acid dissociates completely in water — HCl leaves virtually no undissociated molecules. A weak acid, like ethanoic acid, only partially ionises, so an equilibrium exists between the acid and its ions. Two acids at the same concentration can therefore give very different pH values — that's the mistake examiners see most.

Brønsted-Lowry defines an acid as a proton (H+) donor and a base as a proton acceptor — this covers most SL and HL questions. The Lewis theory, examined only at HL, is broader: a Lewis acid accepts an electron pair and a Lewis base donates one, which explains species like BF₃ that have no proton to give.

For a strong monoprotic acid, pH = -log₁₀[H+], and because dissociation is complete, [H+] equals the acid's stated concentration. So 0.01 mol dm⁻³ HCl gives [H+] = 0.01, and pH = -log₁₀(0.01) = 2. Weak acids can't use this shortcut — you need the Ka expression instead.

Ka is the acid dissociation constant — a measure of how far a weak acid ionises at equilibrium. pKa = -log₁₀(Ka), and a lower pKa means a stronger acid. HL students rearrange Ka = [H+][A-]/[HA] to find pH, [H+], or the equilibrium concentration of a weak acid given its Ka value.

A buffer resists pH change because it contains both a weak acid and its conjugate base — add acid, the conjugate base mops up the extra H+; add base, the weak acid replaces it. HL students calculate buffer pH with the Henderson-Hasselbalch equation: pH = pKa + log₁₀([A-]/[HA]).

Pick an indicator whose colour-change range falls within the steep, near-vertical section of the pH curve at the equivalence point — the equivalence point isn't always pH 7. Phenolphthalein (pH 8.3–10) suits strong acid–strong base and weak acid–strong base titrations; methyl orange (pH 3.1–4.4) suits strong acid–weak base titrations.

Acids and bases appear across all three papers — multiple-choice questions on pH and Kw in Paper 1, extended calculation and definition questions in Paper 2, and data-based questions using real titration curves or buffer data in Paper 3. According to the IB Chemistry guide (first exams 2025), the topic sits under Reactivity 3.1, Proton transfer reactions.

Yes — Paper 3 regularly draws on acid-base practical work, especially titration curves and buffer calculations at HL, because the topic suits graph-reading and error-analysis questions. Expect prompts asking you to sketch or interpret a pH curve, identify the equivalence point, or explain buffer action using given experimental data.

Yes — pH calculations involve logarithms (pH = -log₁₀[H+]), so you'll need your GDC, and the IB permits calculators on all three Chemistry papers. Still, practise recognising simple whole-number cases without one: [H+] = 1 × 10⁻³ mol dm⁻³ gives pH = 3 by inspection, which saves time under exam pressure.

The jump from SL's descriptive pH work to HL's Ka/Kb algebra, buffer equations and titration curve sketching catches a lot of students out — it's the first topic where equilibrium maths meets qualitative theory. In my experience, the real stumbling block isn't the concept, it's misapplying Henderson-Hasselbalch when acid and conjugate base concentrations aren't equal.

Nail three things: know your definitions cold (Brønsted-Lowry, conjugate pairs, Lewis acids), drill pH/pOH/Ka calculations until the algebra is automatic, and learn to read a titration curve — equivalence point, half-equivalence point, buffer region. Working through past Reactivity 3.1 questions is the fastest way to find where your algebra actually breaks down.

IB HL Chemistry covers similar ground to A Level — Ka, pKa, buffers, titration curves — but folds it into the IB's cross-topic Reactivity strand rather than a standalone acids-and-bases unit, and pairs the theory with internal assessment practical work. A Level boards (AQA, Edexcel) often treat buffers and curves as a more isolated, calculation-heavy unit.

Look for resources built around the actual IB Chemistry guide's command terms and mark schemes, not generic chemistry content — topic-specific worksheets with real calculation practice, concise notes covering both SL and HL content, and mock questions styled like Paper 2 and Paper 3. On RevisionPrep, the Chemistry question bank and Topical Worksheets cover Reactivity 3.1–3.4 with full worked solutions.