The same holds for molecules
The same fact holds for molecules. Weigh out the relative molecular mass in grams and, whatever the substance, you have the same number of molecules.
| Substance | Mr | Mass taken | Number of molecules |
|---|---|---|---|
| Hydrogen (H2) | 2 | 2 g | 6.022 × 1023 |
| Ammonia (NH3) | 17 | 17 g | 6.022 × 1023 |
| Carbon dioxide (CO2) | 44 | 44 g | 6.022 × 1023 |
| Sulphur dioxide (SO2) | 64 | 64 g | 6.022 × 1023 |
| Chlorine (Cl2) | 71 | 71 g | 6.022 × 1023 |
| Sulphuric acid (H2SO4) | 98 | 98 g | 6.022 × 1023 |
| Glucose (C6H12O6) | 180 | 180 g | 6.022 × 1023 |
Molecules, or atoms?
18 g of water holds 6.022 × 1023 molecules — not atoms. A water molecule has three atoms in it, so the number of atoms is three times larger. Whenever you read this table, check which of the two you have been asked for.
So one rule covers everything
For atoms, the relative atomic mass in grams; for molecules, the relative molecular mass in grams; for ionic compounds, the formula mass in grams. In all three the number of particles you get is the same.