Chapter summary
The Mole Concept and Stoichiometry, at a glance
Turn masses, gas volumes and solution measurements into amounts, then let a balanced equation connect them.
A-Level 9476 (2026-2027)
Quick revision
Revisit the essentials, then return to an explanation when you need it.
| Term | Definition to write |
|---|---|
| Relative atomic mass, Ar | The weighted average mass of one atom of an element, relative to 1/12 of the mass of one atom of carbon-12. |
| Relative isotopic mass | The mass of one atom of a particular isotope, relative to 1/12 of the mass of one atom of carbon-12. |
| Relative molecular mass, Mr | The weighted average mass of one molecule of a substance, relative to 1/12 of the mass of one atom of carbon-12. Use relative formula mass for an ionic compound. |
| Mole | The amount of substance that contains 6.02 × 1023 elementary particles (the Avogadro constant, L). |
- Identify what was actually measured
Mass, dry gas, a titre, an aliquot or a loss on heating; write units.
- Convert to moles and apply the equation
Check the limiting reagent, coefficients, atom balance and charge balance.
- Return to the requested quantity
Account for dilution, unreacted excess and significant figures; check whether the result is physically plausible.
Formula from mass: divide by atomic molar masses. Combustion: n(C) = n(CO2), n(H) = 2n(H2O). A gas-volume ratio equals a mole ratio only at matching temperature and pressure. An aliquot changes amount, not the concentration of the solution it came from.
Quick recall
Define relative atomic mass.