Stoichiometry: 1.2 - The mole concept


The mole is a fixed number of particles and refers to the amount, n, of substance.

Masses of atoms are compared on a scale relative to 12C and are expressed as relative atomic mass (Ar) and relative formula/molecular mass (Mr).

Molar mass (M) has the units g mol-1.

The empirical formula and molecular formula of a compound give the simplest ratio and the actual number of atoms present in a molecule respectively.

Applications and skills

Calculation of the molar masses of atoms, ions, molecules and formula units.

Solution of problems involving the relationships between the number of particles, the amount of substance in moles and the mass in grams.

Interconversion of the percentage composition by mass and the empirical formula.

Determination of the molecular formula of a compound from its empirical formula and molar mass.

Obtaining and using experimental data for deriving empirical formulas from reactions involving mass changes.


The value of the Avogadro's constant (L or NA) is given in the data booklet in section 2 and will be given for paper 1 questions.

The generally used unit of molar mass (g mol-1) is a derived SI unit.