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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

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.