Moles and masses
The quantitative relationships in the syllabus; unmarked ones are for everyone. Core candidates do reacting-mass calculations in simple proportions only, without the mole.
| Quantity | Formula |
|---|---|
| Relative formula mass | Mr = sum of the Ar of all atoms in the formula |
| Amount of substance Extended | moles (mol) = mass (g) ÷ molar mass (g/mol) |
| Number of particles Extended | particles = moles × 6.02 × 10²³ |
| Gas volume at r.t.p. Extended | volume (dm³) = moles × 24 |
| Relative atomic mass from isotopes Extended | Ar = Σ(isotope mass × % abundance) ÷ 100 |
Solutions
| Quantity | Formula |
|---|---|
| Concentration (mol/dm³) Extended | concentration = moles ÷ volume (dm³) |
| Concentration (g/dm³) | concentration = mass (g) ÷ volume (dm³) |
| Volume conversion | dm³ = cm³ ÷ 1000 |
Percentages
| Quantity | Formula |
|---|---|
| Percentage yield Extended | (actual yield ÷ theoretical yield) × 100 |
| Percentage purity Extended | (mass of pure substance ÷ mass of impure sample) × 100 |
| Percentage by mass of an element Extended | (Ar × number of atoms ÷ Mr) × 100 |
Energy and chromatography
| Quantity | Formula |
|---|---|
| Enthalpy change from bond energies Extended | ΔH = energy to break bonds − energy released making bonds |
| Exothermic reaction Extended | ΔH is negative |
| Rf value Extended | Rf = distance moved by substance ÷ distance moved by solvent |
Finding an empirical formula
- Write the mass (or percentage) of each element.
- Divide each by its relative atomic mass, Ar, to get moles.
- Divide every answer by the smallest one.
- Round to whole numbers (multiply them all if you get halves) to give the simplest ratio.
Empirical and molecular formulae are Extended content.
How to use this sheet
- Write the formula, then substitute, in every calculation: method marks are given for working.
- Convert cm³ to dm³ before using concentration in mol/dm³: divide by 1000.
- Learn the Avogadro constant and molar gas volume; don't rely on them being printed on the paper.
- Source: Cambridge IGCSE Chemistry 0620 syllabus for 2026–2028 (PDF).