5 mole–stoichiometry traps in IB Chemistry Reactivity 2.1
IB Chemistry Reactivity 2.1 – How much? Stoichiometry mastery for Paper 1 and Paper 2, with worked mole ratio, limiting reagent, and gas volume methods.
Volkan Demir2 min read
Note (July 2026): This article was written against the pre-2025 IB Chemistry syllabus. From the May 2025 examination session a new guide is in force: Paper 3 and the option units were removed and the content was reorganised into Structure 1-3 and Reactivity 1-3. The article is kept online for reference. For the current syllabus and exam structure see IB Chemistry tutoring.
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Frequently asked questions
What is the difference between IB Chemistry Reactivity 2.1 and Reactivity 1.4?
Reactivity 1.4 introduces the mole concept and the Avogadro constant as standalone ideas. Reactivity 2.1 extends those ideas into chemical change: balancing equations, applying mole ratios, identifying limiting reagents, and converting between mass, volume, concentration, and gas. In exam conditions, 1.4 appears in Paper 1 multiple choice; 2.1 appears in Paper 1 and the Paper 2 calculation questions.
How much of IB Chemistry Paper 2 is Reactivity 2.1?
On a typical IB Chemistry Paper 2, Reactivity 2.1 contributes roughly four to six marks in Section A and a further two to three marks embedded in the Section B extended-response questions. On Paper 1, two to three multiple-choice questions test the sub-topic. The total weight on the final grade is therefore larger than the syllabus bullet-point count suggests.
Should HL candidates use 22.7 dm³ mol⁻¹ for gas volume calculations?
SL candidates use 22.7 dm³ mol⁻¹ at 273 K. HL candidates are expected to use the ideal gas equation pV = nRT for non-standard conditions, and the marking schedule penalises the use of 22.7 dm³ mol⁻¹ when the temperature is not 273 K. The HL Paper 2 will specify the temperature, and the candidate must apply pV = nRT.
What is the fastest way to prepare for Reactivity 2.1 in IB Chemistry?
Drill the four calculation families — mole-to-mass, limiting reagent, titration, and gas volume — using a method-driven approach rather than memorising past-paper answers. Write the balanced equation on the page every time, show the mole ratio, perform the arithmetic, and quote the answer with units and the correct number of significant figures.
Does the IB Chemistry internal assessment mark Reactivity 2.1?
Yes, the IA processing-of-raw-data criterion rewards the same calculation discipline tested in Paper 2. A candidate who has rehearsed the four-step method in exam conditions will produce a stronger IA data table and propagate uncertainties more accurately. The connection between IA and Paper 2 is direct on Reactivity 2.1.