Chemistry study notes
Chemical Calculations
Turn formulae, measurements and equations into defensible numerical answers.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
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5 topicsRead and construct formulae
Subscripts count atoms; ion charges determine ratios.
Balance atoms, then account for ions
A reaction changes arrangement, not the total number of atoms.
The mole is a counting unit
Connect microscopic particles to measurable mass.
Use the equation ratio
Convert to moles before comparing reactants.
Concentration and titration
Volume tells you moles only when concentration is known.
Scope
Learning outcomes
4. Chemical Calculations (5086 / 5088 / K326 / K328). Use the outcome map to find the explanation for a particular syllabus requirement.
See the learning outcome map
4.1(a) Use chemical symbols and formulae
- Element symbols
- Formulae of named syllabus compounds
4.1(b) Translate atom counts and formulae
- Subscripts, brackets and coefficients
- Reverse interpretation
4.1(c) Balance ion charges to obtain formulae
- Simple and polyatomic ions
- Deduce ion charge from formula
4.1(d) Interpret equations and states
- s, l, g and aq
- Coefficients as mole ratios
4.1(e) Construct balanced equations
- Full equations with states
- Ionic equations and spectators
- Conserve atoms and charge
4.2(a) Define relative atomic mass
- Carbon-12 comparison
- Average atomic mass
4.2(b) Define and calculate relative molecular/formula mass
- Sum relative atomic masses
- Molecular versus ionic terminology
4.2(c) Define the mole using the Avogadro constant
- Specified entities
- Mass-mole-particle relationships
4.2(d) Calculate reacting masses and gas volumes
- Balanced mole ratios
- 24 dm3 per mol at room temperature and pressure
- Limiting reactants
- No variable-temperature gas laws
4.2(e) Use solution concentration
- mol/dm3 and g/dm3
- Titration data
- Unfamiliar reactions with supplied information