The mole - Higher

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

In everyday use, the word 'amount' usually means mass or volume. In chemistry, the word is used differently. The amount of a substance is a measure of how many particles it contains.

Avogadro constant

The Avogadro constant is the number of carbon atoms in exactly 12 g of carbon-12, 12C. It is a very large number:

curriculum-key-fact
The Avogadro constant = 6.02 × 1023

The Avogadro constant is so large because atoms are tiny and have very little mass, so 12 g of carbon consists of a very large number of atoms.

Defining a mole

The mole is the unit for amount of substance. One mole, 1 mol, of a substance can be defined as either:

For example, 1 mol of water molecules is 6.02 × 1023 water molecules. The relative formula mass of water is 18, so 18 g of water contains 6.02 × 1023 molecules.

Calculating the number of particles

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

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

So, number of particles = 6.02 × 1023 × mol

Example

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

number of water molecules = 6.02 × 1023 × 0.5

= 3.01 × 1023

It is important to be clear about the particles involved. For example, 3.01 × 1023 water molecules contain 9.03 × 1023 atoms. This is because a water molecule, H2O, contains three atoms.

Question

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

Number of atoms = 2 × 0.5 × 6.02 × 1023

= 6.02 × 1023