The mole - Higher

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

It is often useful in chemical calculations to know the number of atoms or molecules present. The mole, mol, is the unit for the amount of substance.

Avogadro constant

One mole contains the same number of particles as there are atoms in 12 g of carbon-12. As atoms and molecules are so small, the number of particles in a mole is very large. This number is:

  • called the Avogadro constant
  • equal to 60,000,000,000,000,000,000,000 (6.0 × 1023) atoms, molecules or formula units
curriculum-key-fact
The Avogadro constant = 6.0 × 1023

Calculating the number of particles

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

  • the Avogadro constant, 6.0 × 1023
  • the number of moles

number of particles = 6.0 × 1023 x number of moles

Example

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

Number of water molecules = 6.0 × 1023 x 0.5

= 3.0 × 1023

It is important to be clear about the particles involved. For example, 3.0 × 1023 water molecules contain 9.0 × 1023 atoms. This is because a water molecule, H2O, contains three atoms.

Question

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

Number of atoms = 2 × the number of molecules

Number of molecules = 0.5 × 6.0 × 1023

So number of atoms = 2 × 0.5 × 6.0 × 1023 = 6.0 x 1023

Question

Give the answer in standard form.

3 x 6.0 x 1023 = 18 × 1023

Standard form has a single digit multiplied by the power of ten, so in standard form the number of molecules is 1.8 × 1024.