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Mapped to Exams: IB Chemistry Acids and Bases in Three Weeks

Mapped to Exams: IB Chemistry Acids and Bases in Three Weeks

12 min readOliver Kidd (Co-founder)1 Oct 2026

In IB Chemistry, acids and bases are best handled through the Brønsted–Lowry model of proton transfer, with Arrhenius and Lewis theories kept as supporting definitions for specific question types. Build outward from there to pH calculations, Ka and pKa, buffers and neutralisation. This guide follows that order, noting where SL and HL expectations differ, so your revision moves in the same sequence examiners expect.

TL;DR:

  • Strong acids like HCl completely ionize in water, while weak acids such as acetic acid only partially ionize, requiring Ka and pKa to measure their strength.
  • Conjugate acid-base pairs differ by one proton, with amphiprotic species capable of both donating and accepting protons depending on the reaction context.
  • Arrhenius theory primarily applies to aqueous H⁺ or OH⁻ reactions, whereas Brønsted–Lowry covers proton transfer reactions in broader conditions, including non-water environments.
  • Buffer solutions resist pH changes through the ratio of conjugate base to acid, which must be accurately calculated using Henderson-Hasselbalch, especially at high ionic strength where activity coefficients matter.
  • Use targeted practice tools, mock exams, and exam-level questions from examiner-created resources for systematic preparation, with special focus on weak acid/base equilibrium and buffer calculations.
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Table of Contents

Arrhenius, Brønsted–Lowry and Lewis: choosing the right model

Three theories describe acids and bases, and IB questions expect you to know which one applies. The Arrhenius definition, an acid produces H⁺ ions and a base produces OH⁻ ions in water, works well for simple aqueous reactions but says nothing about non-aqueous chemistry. The Brønsted–Lowry model, where acids donate protons and bases accept them, is the core IB framing and covers most syllabus content, including reactions without water. The Lewis definition, an acid accepts an electron pair and a base donates one, extends the idea further and explains reactions such as boron trifluoride accepting a lone pair from ammonia.

Use this quick check before answering:

  • If the question mentions proton transfer or conjugate pairs, use Brønsted–Lowry.
  • If it involves a species with no protons to donate, such as a metal ion or BF₃, think Lewis.
  • If the reaction only concerns H⁺ or OH⁻ ions in water, Arrhenius language is enough.

How to identify conjugate acid-base pairs and amphiprotic species

A conjugate pair differs by a single proton: HA loses one to become A⁻, and a base B gains one to become BH⁺. Spotting these pairs quickly is one of the more reliable mark-earning habits in IB Chemistry.

  1. Write the full equation and underline the species losing a proton and the one gaining it.
  2. Label each reactant and its product as a conjugate pair, acid on one side and base on the other.
  3. Check for amphiprotic species: these can both donate and accept protons, such as HCO₃⁻ or H₂O.
  4. Distinguish amphoteric from amphiprotic: amphoteric species react with both acids and bases but do not necessarily do so through proton transfer, as with aluminium hydroxide.

Try this yourself: identify the conjugate pairs in the reaction between HSO₄⁻ and water, then decide whether HSO₄⁻ is acting as an acid or a base in that instance.

Strong versus weak acids and bases, Ka and pKa explained

Strength refers to the extent of ionisation, not concentration. A strong acid or base ionises completely in water, while a weak one establishes an equilibrium, only partially ionising. Ka measures that equilibrium position for a weak acid: a large Ka means more ionisation and a stronger acid, while pKa (the negative log of Ka) moves the other way, so a lower pKa signals a stronger acid.

  • Strong acids and bases: full ionisation, no equilibrium expression needed.
  • Weak acids and bases: partial ionisation, described by Ka or Kb.
  • A stronger acid has a weaker conjugate base, and vice versa.
  • Before solving for x in an equilibrium expression, check whether the approximation (Ka small compared with initial concentration) is valid.

A Ka value below roughly 10⁻³ typically signals that the “x is small” approximation is safe to use, according to the subject brief’s guidance on method marks for equilibrium problems: examiners want to see that check stated, not just assumed.

Step-by-step pH calculations and buffer capacity

Different acid and base types need different calculation routes, and knowing which one to reach for saves time under exam pressure.

  1. Strong acid or base: pH = negative log of [H⁺] directly, or find [OH⁻] first and convert via pOH.
  2. Weak acid or base: set up the Ka or Kb expression, solve for x (the H⁺ or OH⁻ concentration), and verify the small-x assumption before finalising your answer.
  3. Buffer pH: use the Henderson-Hasselbalch equation, pH equals pKa plus the log of the ratio of conjugate base to acid concentration, as set out in OpenStax’s buffer chapter.
  4. Buffer capacity: the amount of acid or base a buffer can absorb before pH shifts significantly, which depends on how much conjugate acid and base are present, not just their ratio.

Pro Tip: Always state units and check your ratio the right way up in Henderson-Hasselbalch: mixing up acid and base terms is one of the most common lost marks on buffer questions.

A frequent pitfall at HL is treating Henderson-Hasselbalch as exact at all concentrations. The equation assumes concentrations rather than activities, so it becomes less reliable in solutions with high ionic strength, a nuance worth mentioning if a question probes it.

Writing neutralisation equations and naming the salt formed

Neutralisation reactions between an acid and a base produce a salt and, in most cases, water. Writing the ionic equation clearly, and stripping out anything that does not actually react, earns marks examiners are specifically checking for.

  • Write the full molecular equation first, then the ionic equation showing dissociated species.
  • Cancel spectator ions (those unchanged on both sides) to leave the net ionic equation.
  • Identify the salt from the cation of the base and the anion of the acid.
  • With a carbonate, expect carbon dioxide gas alongside water and the salt, since carbonic acid decomposes.

For mole-ratio practice in these reactions, our stoichiometry guide walks through the calculations examiners pair with neutralisation questions.

Worked examples and examiner advice for exam-style questions

Short, fully annotated examples make the method visible, which is exactly what mark schemes reward.

  1. Strong acid pH: for 0.010 mol dm⁻³ HCl, [H⁺] = 0.010, so pH = 2.00 (two significant figures, matching the data given).
  2. Weak acid pH: for a 0.100 mol dm⁻³ acid with Ka = 1.8 × 10⁻⁵, x² ÷ 0.100 ≈ Ka, giving x ≈ 1.34 × 10⁻³, so pH ≈ 2.87 after confirming x is small relative to 0.100.
  3. Buffer pH: with pKa = 4.76 and equal concentrations of acid and conjugate base, pH = pKa = 4.76 directly from Henderson-Hasselbalch.

Label every conjugate pair you use, show the equilibrium expression before substituting numbers, and state your small-x check explicitly.

These habits align closely with how IB mark schemes allocate method marks, not just final-answer marks. Our equilibrium walkthrough covers the same small-x logic in more depth for Kc problems.

Where to practise acids and bases on Tiber Tutor

Reading through theory only gets you so far. Pairing it with targeted practice and visual tools closes the gap between recognising a concept and applying it under exam conditions.

  • Interactive tools such as the PhET acid-base simulation let you test intuition about ionisation and buffer behaviour before tackling calculations.
  • Topic tests on acids and bases isolate weak spots quickly, so revision time goes where it is needed.
  • Mock exams apply the same concepts under timed, syllabus-matched conditions.
  • Cross-check SL versus HL requirements in our syllabus guide before deciding how deep to go on buffer calculations.

A compact revision plan for acids and bases

Three weeks is enough if the mix is right. Spend week one on concept review: theories, conjugate pairs, Ka and pKa. Week two moves to worked problems, pH and buffer calculations done by hand until the steps feel automatic. Week three is timed practice, using topic tests and a mock exam to surface anything still shaky, then returning to that specific gap rather than restarting the whole topic.

— Oliver

Get exam-ready with a platform built by IB examiners

Revision notes explain the theory, but confidence comes from seeing exactly where your understanding breaks down. Tiber Tutor is built by IB examiners, so every topic test, mock exam and worked answer on acids and bases mirrors how the real paper is marked, not just how the syllabus reads. The platform links each acids and bases resource, videos, notes, flashcards and question banks, to the same progress tracking system, so a weak Ka calculation on a topic test flags itself in your analytics rather than resurfacing as a surprise in a mock exam.

Get exam-ready with a platform built by IB examiners — overview diagram

That combination, examiner-authored content plus performance tracking in one place, is not something revision sites or generic question banks offer together. Start with the IB Chemistry topic tests to check your grasp of pH and buffers, then move to a full mock exam once the analytics show you are consistently scoring well. Plans start with a free 7-day trial, after which the All-Access Plan and Per-Subject Plan are available from the pricing page.

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FAQ

What is the difference between Arrhenius and Brønsted-Lowry acids?

Arrhenius acids produce H⁺ in water specifically, while Brønsted-Lowry acids donate a proton in any reaction, aqueous or not. IB Chemistry uses Brønsted-Lowry as its core model because it covers a wider range of reactions, including those without water present.

How do you know if a species is amphiprotic?

A species is amphiprotic if it can both donate and accept a proton, depending on what it reacts with. Common examples include HCO₃⁻ and water itself, both of which appear frequently in IB-style equilibrium and neutralisation questions.

What does a small Ka value tell you about an acid?

A small Ka value means the acid only partially ionises in water, making it a weak acid, and its conjugate base is correspondingly stronger. This relationship is central to predicting pH and to justifying the small-x approximation in equilibrium calculations.

How do you calculate buffer pH using Henderson-Hasselbalch?

Buffer pH equals pKa plus the logarithm of the ratio of conjugate base concentration to acid concentration, as explained in OpenStax’s buffer chapter. The equation assumes concentrations rather than activities, so it is less precise at high ionic strength.

Does Tiber Tutor cover both SL and HL acids and bases content?

Yes, Tiber Tutor’s chemistry resources are organised by syllabus level, so acids and bases material is separated into SL and HL requirements. Topic tests, videos and mock exams are written by IB examiners and mapped directly to the current subject guide.