The mole: weigh a pure sample, then count the moles, the molecules and the atoms inside it

Chemists count atoms with a scale

Atoms are far too small and far too many to count one by one, so chemists weigh them instead. One mole is 6.02 × 10²³ particles, and a mole of any substance has a mass in grams equal to its relative mass. Raise the sample mass and watch the moles, molecules and atoms climb together.

Counting by weighing H₂O · M = 18.0 g/mol
Pick a substance. Raise the mass. Compare particles with atoms.
1.00 mol · 6.02×10²³ molecules
Sample mass18.0 g
Amount (n)
1.00 mol
Molecules (N)
6.02×10²³
Take it further

Know This

One mole is always 6.02 × 10²³ particles, so dividing a weighed mass by molar mass, n = m/M, tells you how many moles, and so how many atoms or molecules, are in the sample.