MYP Chemistry: The Mole & Basic Stoichiometry — FAQ

A mole is simply a counting unit — like a dozen, but for atoms and molecules. One mole of any substance contains 6.02 × 10²³ particles, a number called the Avogadro constant. In MYP Chemistry you use moles to convert between the mass you weigh on a balance and the number of particles actually reacting.

You calculate moles using n = m ÷ M, where n is moles, m is the given mass in grams, and M is the molar mass in grams per mole. Find M by adding up the atomic masses of every atom in the formula from the periodic table, then divide your mass by it.

Avogadro's constant is 6.02 × 10²³ — the number of particles in exactly one mole of any substance, whether that's atoms, molecules or ions. It matters because it bridges the microscopic world of individual atoms, which you can't count directly, with the macroscopic world of grams, which you can measure on a lab balance.

Balancing means adjusting the number in front of each formula until the atoms of every element match on both sides — you never change a subscript inside a formula. Once balanced, the coefficients give you the mole ratio between reactants and products, which is the entire point of a stoichiometry calculation at this level.

Your teacher assesses mole and stoichiometry work mainly against MYP Sciences Criterion A (Knowing and Understanding) and Criterion C (Processing and Evaluating), each marked out of 8. You'll apply the mole formula, interpret data from a practical, and calculate quantities — then explain your reasoning, not just state a final number.

Mole calculations sit mostly under Criterion A, where you apply scientific knowledge and formulae, and Criterion C, where you process data and reach conclusions using calculations. According to the IB's MYP: From Principles into Practice guide, all four Sciences criteria are assessed at least twice across the year, so stoichiometry can resurface in a lab report too.

Expect 'calculate' (get a numerical answer, showing full working), 'determine' (find a value using given information), 'deduce' (draw a conclusion from data provided) and 'state' (give a brief answer, no explanation needed). The command term tells you exactly how much working and justification the marker is expecting to see.

The single most common error I see marking mock papers is forgetting units — writing '0.25' instead of '0.25 mol'. Close behind: using atomic mass instead of molar mass for compounds, and getting the mole ratio backwards when scaling stoichiometry answers up or down.

A genuine 7 needs more than a correct number — Criterion A rewards explaining why you chose a particular formula, and Criterion C rewards evaluating whether your data or answer is actually reliable. Show full working, state units at every step, and always sanity-check whether your final answer is a realistic size.

It's conceptually odd at first because you're working with a number too large to picture, but the maths itself is straightforward once you're comfortable rearranging n = m ÷ M. Most students who struggle are actually stuck on basic algebra, not chemistry — practising the rearrangement on its own usually fixes it within a week or two.

MYP gives you the foundation formula n = m ÷ M and the Avogadro constant; DP Chemistry builds directly on top of it. According to the IB, the current Chemistry guide (first exams 2025) opens with Structure 1.4 on the mole, extending straight into molar concentration and gas volume calculations you'll meet in Year 1 of the Diploma.

The core idea is identical, but DP goes further quantitatively — adding limiting reagent calculations, percentage yield, molar gas volume, and titration stoichiometry that MYP only introduces conceptually. Think of MYP as building the toolkit; DP hands you the full workshop of applications built from that same one formula.

Look for resources pairing short concept notes with lots of practice questions — the mole is a maths-heavy topic mastered through repetition, not memorised through reading alone. On RevisionPrep, MYP Chemistry Revision Notes and Topical Worksheets on the mole let your child drill the n = m ÷ M formula until the rearrangement stops being the sticking point.

Sit with your child while they show you the working, not just the final answer — most mole mistakes are unit or rearrangement errors that are obvious once written down step by step. Ask them to explain out loud why they picked a particular formula; if they can't, that's the gap to fix before the next mock.