Atoms and molecules are far too small to count one at a time, so chemists count them in huge groups called moles. One mole of any substance contains 6.02 × 1023 particles. This number is the Avogadro constant. The particles can be atoms, molecules or ions, depending on the substance. One mole of water contains 6.02 × 1023 water molecules, and half a mole (0.5 mol) contains 3.01 × 1023.
The mole was first defined using carbon-12: one mole is the number of atoms in exactly 12 g of carbon-12. The newer definition fixes the mole as exactly the Avogadro number of particles. OCR accepts both. In grams, the relative atomic mass or relative formula mass of a substance is the mass of one mole of it. So 12 g of carbon and 18 g of water each contain 6.02 × 1023 particles.
You can find the mass of one atom or molecule by dividing the mass of one mole by the Avogadro constant. For carbon, 12 ÷ (6.02 × 1023) = 2.0 × 10-23 g. For one water molecule, 18 ÷ (6.02 × 1023) = 3.0 × 10-23 g. The answers are written in standard form.