MYP · Chemistry MYP Year 4
Stoichiometry & Mole Concept
24 min readStandardCore graded unit — assessed through MYP Criterion B/C investigation tasks and calculation-heavy short-answer questions; regularly framed around real industrial contexts (e.g. Haber process, emissions data) rather than pure numbers.
Every question in this chapter is really asking the same thing: how many particles am I actually working with? The mole is just a very large counting number with a name, and once you can move fluently between mass, moles and particles, every 'calculate the mass/concentration/yield of...' question collapses into the same three-line routine — balanced equation, mole ratio, molar mass. Most marks lost here aren't conceptual; they're unit slips (cm³ vs dm³, tonnes vs grams) and forgetting to convert to moles before touching a ratio.
Overview — The Shape of the Chapter
Why chemists count in 'moles' at all
- Atoms and molecules are absurdly small and light — a single water molecule has a mass of about g, far too small to weigh on any real balance.
- The mole fixes this by defining an enormous, exact number of particles () as 'one unit' — like calling 12 eggs 'a dozen', except this number is chosen so one mole of atoms has a mass in grams numerically equal to its relative atomic mass.
- Every calculation in this chapter — however dressed up with industrial plants or lab balances — is just converting between mass, moles and number of particles (or volume/concentration for solutions and gases).
The shape of the chapter
Command terms this topic loves to test
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| State | Give a brief, precise fact, value or unit — no working required. | AO1 | 1 mark needs the value AND its unit together — a correct number with no unit typically scores zero. |
| Calculate | Obtain a numerical answer, showing the working that leads to it. | AO2 | Marks sit on the method steps (formula, correct substitution) as well as the final answer — an unsupported correct number can still lose method marks. |
| Determine | Obtain a value from given data, often needing an extra decision first (e.g. which reagent is limiting). | AO2/AO3 | Expect an intermediate step to be identified explicitly before the final number is credited. |
| Deduce | Reach a conclusion using reasoning from the data given, not a recalled fact. | AO3 | The justification sentence carries its own mark — a correct number with no reasoning line loses it. |
| Explain | Give a reason or mechanism, linking cause to effect ('because...', 'this means...'). | AO3 | A bare unlinked statement scores zero for 'explain' — the causal link is what's actually marked. |
| Identify | Name or select the correct item from given information. | AO1 | Usually 1 mark for the precise correct term — no explanation expected unless the question also says 'and explain'. |
Key point
The mole is a counting unit, not a mass or a substance — 'one mole of X' always means particles of X, whatever X is. Every other equation in this chapter is arithmetic built on that single fact.
Overview