IB Chemistry: Balancing Equations & Stoichiometry FAQ

You need three things solid: balancing equations by inspection, the mole concept ($n=m/M$), and converting between moles, mass, volume and concentration. According to the IB, this sits under Structure 1.4 in the 2025 Chemistry guide — 'Counting particles by mass: the mole' — and it underpins calculations across the whole course.

Balance the atom that appears in the fewest species first, then work outward, saving oxygen and hydrogen for last. Never change subscripts — only add coefficients in front of formulae. Check every element, then check charge balance if it's an ionic equation. It's mechanical once you've done a dozen.

A mole is 6.02 × 10²³ particles — Avogadro's constant — and it's the bridge between the tiny (atoms, molecules) and the measurable (grams, litres). Every stoichiometry calculation in IB Chemistry, from titrations to gas volumes, is really just converting a balanced equation's mole ratios into real-world quantities.

An empirical formula gives the simplest whole-number ratio of atoms in a compound; a molecular formula gives the actual number. IB questions usually give you mass or percentage composition data, ask you to find moles of each element, divide by the smallest, then scale up using the given molar mass.

Paper 1 tests it through multiple-choice questions on mole ratios, gas volumes and concentration — quick, calculator-free reasoning. Paper 2 has full calculation questions, often worth 4-8 marks, requiring working shown for method marks even if your final answer is wrong.

The core mole-ratio maths is identical at SL and HL — the difference is what's layered on top. HL students meet more multi-step calculations combining stoichiometry with equilibrium constants (Reactivity 2) and more complex titration or back-titration scenarios, which is where marks are actually lost.

A single balancing task is usually worth 1 mark, but full stoichiometry calculations in Paper 2 commonly run 3-6 marks per question, sometimes stretching to 8 when combined with percentage yield or titration data. Across a full exam paper, stoichiometry-linked marks often add up to 15-20% of the total.

Yes, indirectly and often. Most IA titration, gravimetric or gas-collection investigations require you to calculate moles, determine limiting reagent, or find percentage yield to process your raw data — this sits under the 'Analysis' criterion, not a separate stoichiometry mark.

Convert both reactant masses to moles, divide each by its coefficient in the balanced equation, and whichever gives the smaller value is the limiting reagent. Use that reagent's moles to calculate the theoretical yield of product — the other reactant is in excess and irrelevant to yield.

The biggest one, in fifteen years of marking mocks, is using the wrong molar mass — forgetting hydrates' water of crystallisation, or misreading a subscript. Second is skipping the limiting reagent check entirely and just using whichever reactant's number looks easier to work with.

Drill the mole-conversion chain (mass ↔ moles ↔ volume ↔ concentration) until it's automatic, not something you re-derive each time. Then work through past-paper Paper 2 stoichiometry questions specifically, checking your working against the mark scheme's method marks, not just the final answer.

Past IB papers are the gold standard because mark schemes show exactly how method marks are awarded. Beyond that, topic-specific worksheets that isolate mole calculations, limiting reagent and titrations separately let a student build fluency in each sub-skill before combining them under exam conditions.

Official IB past papers are free through a school's subscription, but many families supplement with topic-focused question banks, which typically run from roughly USD 10-30 a month depending on subject coverage. The real cost saving is time — targeted stoichiometry practice avoids paying for broad, unfocused tuition.