The mole
Atoms are far too small to count one at a time, so chemists count them in a very large group called the mole (mol).
1 mol = 6.022 × 1023 particles
This number is Avogadro’s number. A mole works like a dozen: a dozen eggs is 12 eggs, and a mole of atoms is 6.022 × 1023 atoms. The particles can be atoms, molecules, ions or formula units.
- moles to particles: multiply by 6.022 × 1023
- particles to moles: divide by 6.022 × 1023
Worked example: moles to particles
How many molecules are in 2.50 mol of water?
2.50 mol × 6.022 × 1023 molecules/mol = 1.51 × 1024 molecules
Worked example: particles to moles
How many moles is 3.01 × 1023 atoms of copper?
3.01 × 1023 atoms ÷ 6.022 × 1023 atoms/mol = 0.500 mol
Common mistake
Forgetting that a mole of molecules contains more than a mole of atoms. One mole of H2O contains 2 mol of hydrogen atoms and 1 mol of oxygen atoms.