IB Chemistry Paper 1A: multiple-choice strategy, timing and the traps to expect

IB Chemistry Paper 1A is the multiple-choice section of Paper 1: 30 questions at SL and 40 at HL, one mark each, with no penalty for wrong answers. On the current course you may use a calculator and you get a clean copy of the data booklet, so questions can include real calculations. The way to score well is to answer the easy questions quickly, estimate instead of calculating in full, recognise the standard distractors, and leave enough time for Paper 1B, which is sat in the same session.

What does Paper 1A look like on the current course?

For how 1A fits alongside 1B and Paper 2, see what the Chemistry exams look like now.

How long should each multiple-choice question take?

Paper 1 gives you about 1.6 minutes per mark at both levels. Split strictly by marks, that is roughly 50 minutes for 1A at SL and 65 minutes at HL. But a multiple-choice mark usually takes less writing than a 1B mark, so a better plan is:

1. First pass at about a minute per question. Answer what you can, and flag any question that needs more than a quick calculation.
2. Second pass on the flagged questions, with a fixed time limit.
3. Move to 1B with the time you have banked. At SL, a first pass of 30 minutes plus 10 minutes on flagged questions leaves about 50 minutes for 1B's 25 marks.

Fill in an answer for every flagged question on the first pass, even a guess, so nothing stays blank if time runs out. More on pacing across a whole paper: time management in IB exams.

How do you work out an answer you're unsure of?

Answer before you look at the options

Cover the options and work out what you expect. If your answer is there, choose it and move on. If you look first, a well-built distractor can talk you into it.

Eliminate

Most questions have one or two options you can rule out in seconds: a wrong sign, impossible units, a pH above 7 for an acid. Crossing them out turns a 25% guess into a 50% one.

Estimate instead of calculating in full

Options are usually far enough apart that rounded numbers are enough. 0.24 g of Mg is about 0.010 mol; 100.09 g mol⁻¹ is about 100. Use the calculator to confirm when two options are close, not as the first step.

Check the units

If the options are in dm³ and your working is in cm³, one of the options will be your answer multiplied or divided by 1000, placed there on purpose.

Use the data booklet for what it's for

Know where to find relative atomic masses, constants, bond enthalpies and the other tables before the exam, so a lookup takes seconds. If a question gives you a value in the stem, use that value, not the booklet's.

Read every word of the stem

Underline NOT, INCORRECT, LEAST and "in aqueous solution". For Roman-numeral questions (I, II, III), decide true or false for each statement separately, then find the matching option.

Which distractor patterns should you expect?

Many of these are the same slips that cost marks in written answers; our list of common IB Chemistry mistakes has the full set with worked fixes.

Which topics generate the most traps?

• Moles and gases (Structure 1.4, 1.5; Reactivity 2.1): ratios, limiting reagents, molar volume, cm³/dm³.
• Shapes and polarity (Structure 2.2): polar bonds in non-polar molecules, lone pairs changing bond angles.
• Periodic trends (Structure 3.1): which way atomic radius, ionisation energy and electronegativity change, and why.
• Isomers and naming (Structure 3.2): counting structural isomers, longest-chain naming.
• Energetics (Reactivity 1.1, 1.2): signs, and bonds broken minus bonds formed.
• Equilibrium (Reactivity 2.3): what changes K (only temperature) and what changes only the position.
• Acids and redox (Reactivity 3.1, 3.2): strong vs concentrated, oxidation states, which species is the oxidising agent.

Five worked questions

Original questions written for this guide; they are not IB questions.

Question 1: estimate, then check units

What volume of CO₂, measured at STP, is produced when 5.0 g of CaCO₃ decomposes completely? CaCO₃ → CaO + CO₂ (M of CaCO₃ = 100.09 g mol⁻¹; molar volume at STP = 22.7 dm³ mol⁻¹)

A. 0.113 dm³
B. 1.13 dm³
C. 2.27 dm³
D. 1130 dm³

B. 5.0 ÷ 100 ≈ 0.050 mol CaCO₃, giving 0.050 mol CO₂ (1:1); 0.050 × 22.7 ≈ 1.1 dm³. D is the same number in cm³ labelled as dm³; C doubles the amount.

Question 2: polar bonds vs polar molecules

Which molecule is polar?

A. CO₂
B. BF₃
C. CCl₄
D. NH₃

D. All four have polar bonds. In the linear, trigonal planar and tetrahedral molecules the bond dipoles cancel. NH₃ is trigonal pyramidal because of its lone pair, so its dipoles do not cancel.

Question 3: what changes K

N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. Which change increases the value of K?

A. Increasing the pressure
B. Adding a catalyst
C. Decreasing the temperature
D. Removing NH₃ as it forms

C. Only temperature changes K. The forward reaction is exothermic, so lowering the temperature favours it and K increases. A and D shift the position to the right without changing K, which is exactly why they are tempting; B changes neither.

Question 4: Roman numerals, strong vs weak

Equal volumes of 0.10 mol dm⁻³ HCl(aq) and 0.10 mol dm⁻³ CH₃COOH(aq) are compared. Which statements are correct?

I. HCl(aq) has the lower pH.
II. Both need the same volume of 0.10 mol dm⁻³ NaOH(aq) for neutralisation.
III. Both have the same electrical conductivity.

A. I and II only
B. I and III only
C. II and III only
D. I, II and III

A. I: HCl dissociates completely, so [H⁺] is higher and pH lower. II: both contain the same amount of a monoprotic acid, so they need the same amount of NaOH. III: the weak acid produces fewer ions, so it conducts less. Judge each statement alone before looking at the options.

Question 5: oxidation states in a polyatomic ion

What is the oxidation state of sulfur in the thiosulfate ion, S₂O₃²⁻?

A. −2
B. +2
C. +4
D. +6

B. 2x + 3(−2) = −2, so 2x = +4 and x = +2. C forgets to divide by the two sulfur atoms; D is sulfur's oxidation state in sulfate, there to catch anyone answering from memory.

How should you practise for Paper 1A?

• Explain every wrong option. After each question, say in one line why each distractor is wrong. That is how you learn the patterns above.
• Time yourself in blocks of ten. Ten questions in ten minutes, with a calculator and the data booklet on the desk.
• Use older papers with care. Pre-2025 multiple-choice questions are still good for content that is on the current syllabus, but they assume no calculator. Our guide to finding IB past papers covers where to get them legitimately.
• Don't neglect the written half. Paper 1B and Paper 2 reward the same calculations, with the working shown.