IGCSE Chemistry 0620 key formulae
Every calculation formula you need for IGCSE Chemistry, from the 2026–2028 syllabus. The Periodic Table in the exam gives relative atomic masses and the molar gas volume (24 dm³ at r.t.p.), but not these formulae.
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Moles and masses
- Amount of substance Extended
- moles = mass (g) ÷ molar mass (g/mol)molMolar mass in g/mol has the same number as Mr (e.g. H₂O: 18 g/mol).
- Relative formula mass
- Mr = sum of the Ar values of every atom in the formulano unit
- Number of particles Extended
- particles = moles × 6.02 × 10²³atoms, ions or molecules
- Percentage composition by mass Extended
- % of element = (Ar × number of atoms ÷ Mr) × 100%
Gases and solutions
- Gas volume at r.t.p. Extended
- volume (dm³) = moles × 24dm³ (× 24 000 for cm³)One mole of any gas is 24 dm³ at room temperature and pressure.
- Concentration Extended
- concentration (mol/dm³) = moles ÷ volume (dm³)mol/dm³
- Concentration in g/dm³
- g/dm³ = mol/dm³ × Mrg/dm³
- cm³ to dm³
- dm³ = cm³ ÷ 1000
Yield and purity
- Percentage yield Extended
- % yield = (actual yield ÷ theoretical yield) × 100%
- Percentage purity Extended
- % purity = (mass of pure product ÷ mass of impure sample) × 100%
Other
- Rf value (chromatography) Extended
- Rf = distance travelled by substance ÷ distance travelled by solventno unit, always less than 1
- Energy change from bond energies Extended
- ΔH = energy to break bonds − energy released making bondskJ/mol; negative = exothermic
- Mean rate of reaction
- rate = change in amount (or volume) ÷ time takene.g. cm³/s
Three habits for chemistry calculations
- Always change to moles first, then use the ratio from the balanced equation.
- Change cm³ to dm³ by dividing by 1000 before using a concentration.
- Show the units at every step and give the final answer to the accuracy asked (usually 3 significant figures).
Questions students ask
What is the molar gas volume in IGCSE Chemistry?
24 dm³ per mole for any gas at room temperature and pressure (r.t.p.). It is printed on the data sheet.
Are mole calculations Core or Extended?
Most mole calculations are Supplement (Extended) content. Core includes relative formula mass and concentration units.