The mole - Higher

Introducing the mole, the unit of measurement for the number of particles in a substance

The actual masses of atoms, molecules and ions are too small to be useful in calculations. For this reason, chemical amounts are measured in moles. The mole is a unit. Its symbol is mol.

The mass of one mole of a substance is equal to:

The table shows the masses of one mole of three substances.

SubstanceFormulaRelative formula massMass of 1 mol
CalciumCa4040 g
OxygenO22 × 16 = 3232 g
Calcium carbonateCaCO340 + 12 + (3 × 16) = 100100 g

Avogadro constant

One mole of a substance contains the same number of particles as one mole of any other substance. The particles can be atoms, molecules or ions.

The number of atoms, molecules or ions in one mole of a substance is called the Avogadro constant. Its value is 6.02 × 1023 per mole, which is 602,000,000,000,000,000,000,000 per mole.

The amount in moles can apply to atoms, molecules, ions and electrons. For example, the number of atoms in 1 mol of sulfur is the same as the number of molecules in 1 mol of sulfur dioxide. This is the same as the number of sodium ions in 1 mol of sodium chloride. The number of chloride ions in 1 mol of sodium chloride is also the same.

curriculum-key-fact
One mole, 1 mol, of a substance is the Avogadro constant number (6.02 × 1023) of particles (atoms, molecules, ions or formulae) of that substance.

Calculating the number of particles

The number of particles in a substance can be calculated using:

  • the Avogadro constant (6.02 × 1023)
  • the amount of substance in mol

Number of particles = Avogadro constant × the amount of substance in mol

Example

Calculate the number of water molecules in 0.5 mol of water.

Number of water molecules = Avogadro constant x amount of substance in mol

= 6.02 × 1023 × 0.5

= 3.01 × 1023

It is important to state the particles involved. For example, 3.01 × 1023 water molecules contain 9.03 × 1023 atoms. This is because one water molecule, H2O, is made up of three atoms.

Question

Calculate the number of oxygen atoms in 0.5 mol of oxygen molecules, O2.

Number of atoms = 6.02 × 1023 per mol × 0.5 mol × 2

= 6.02 × 1023