IB Chemistry: Empirical & Molecular Formulae FAQ

Convert given masses or percentages to moles using molar mass, divide each by the smallest mole value to get a simple whole-number ratio, then round sensibly — that ratio is the empirical formula. Multiply it by a whole number found by dividing the molar mass by the empirical formula mass to get the molecular formula.

An empirical formula gives the simplest whole-number ratio of atoms in a compound, while a molecular formula gives the actual number of each atom in one real molecule. Glucose has empirical formula CH₂O but molecular formula C₆H₁₂O₆ — six times the empirical ratio.

When the empirical formula mass equals the molar mass, the ratio n = molar mass ÷ empirical mass equals 1, so the two formulae are identical. Water (H₂O) and carbon dioxide (CO₂) are common examples — small, simple molecules where no scaling up is needed.

Treat the percentages as grams in a 100 g sample, divide each by its atomic molar mass to get moles, then divide every mole value by the smallest one. Round to the nearest whole number — if you get something like 1.5, double all the ratios before rounding.

Find moles of CO₂ to get moles of carbon, and moles of H₂O to get moles of hydrogen (double the water moles, since each molecule has two H atoms). If oxygen is present, find its mass by subtracting the mass of C and H from the total sample mass, then convert to moles.

Divide the given molar mass of the compound by the empirical formula's mass to get a whole number n, then multiply every subscript in the empirical formula by n. If n comes out as a non-whole number, double-check your empirical formula rounding first — that's usually where the error is.

The three big ones I see every year: forgetting to double hydrogen moles from water in combustion analysis, rounding a ratio like 1.33 straight to 1 instead of scaling up, and mixing up mass percentage with mole ratio. Each one throws off the entire final answer even when the method is otherwise sound.

Yes — empirical and molecular formula calculations sit within Structure 1.4 (Counting particles by mass: the mole) and are core content for both SL and HL. According to the IB Chemistry guide, first examined in 2025, this topic is assessed identically at both levels since it's foundational quantitative chemistry, not an HL-only extension.

Expect anywhere from 1 mark (a single multiple-choice item on Paper 1) up to 4-5 marks in a structured Paper 2 question that walks through percentage composition to empirical formula to molecular formula. Marks are typically awarded stepwise for correct moles, correct ratio, and correct final formula.

Yes — the IB Chemistry Data Booklet gives relative atomic masses, so you never need to memorise them. It won't do the mole-ratio steps for you, though, so the real skill being examined is the calculation method itself, not data recall.

Empirical formula questions are really percentage composition and molar mass questions combined into one process — you use percentage composition to find the mole ratio, then use molar mass to scale that ratio up to the true molecular formula. Master one and the others follow naturally.

Empirical formula calculations are unique to Chemistry — Physics and Biology don't test this specific skill, though both use mole and mass concepts elsewhere. Within Chemistry, it's considered a mid-difficulty topic: straightforward once the method is drilled, but easy to lose marks on through careless rounding.

Past paper questions from Paper 1 and Paper 2 are the gold standard, since examiners reuse the same question structures year after year. On RevisionPrep, the Chemistry Topical Worksheets group these calculations by sub-skill (percentage composition, combustion analysis, molar mass scaling) so students can drill one weak step at a time.

The fastest fix is usually just repetition of the five-step method until it's automatic — this isn't a topic that needs deep conceptual re-teaching, it needs practice under time pressure. Ask your child to talk you through one worked example out loud; if they can explain each step, they've usually got it.