Atoms and molecules are far too small to count one at a time, so chemists count them in groups called moles. One mole of any substance contains the same number of particles, and that number is the Avogadro constant. Its value is 6.02 × 1023 per mole. The particles can be atoms, molecules or ions. The classical definition says one mole is the amount of substance that contains as many particles as there are atoms in 12 g of carbon-12. The newer definition is based directly on the Avogadro constant, and OCR accepts both.
The Avogadro constant is a very large number, so it is written in standard form. The number of particles in a sample is the number of moles multiplied by 6.02 × 1023. For example, 2 mol of water molecules contain 2 × 6.02 × 1023 molecules, which is 1.20 × 1024 molecules to three significant figures.
One mole of an element has a mass in grams equal to its relative atomic mass. One mole of carbon atoms therefore has a mass of 12 g. The mass of one atom is found by dividing the mass of one mole by the Avogadro constant: 12 ÷ 6.02 × 1023 = 1.99 × 10-23 g. This is a very small mass. In the same way, one water molecule (relative formula mass 18) has a mass of 18 ÷ 6.02 × 1023 = 2.99 × 10-23 g.