Molecular mass
Water's formula, H₂O, adds up to 18 u.
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The rule
In a molecule's formula, a small number counts atoms. H₂O has 2 hydrogen atoms and 1 oxygen atom.
A molecule is a group of atoms that can exist alone. Molecular mass is the total mass of one molecule, in u.
Some solids are made of ions, not molecules. Ions carry an electric charge. A formula unit is the simplest whole-number ratio of ions.
Add each atom's mass, from the formula's counts. For a molecule, that total is its molecular mass. For ions in a ratio, it is one formula unit's mass.
Add it up
| Formula | Working | Mass |
|---|---|---|
| H₂O | 2 × 1 + 1 × 16 | 18 u |
| NH₃ | 1 × 14 + 3 × 1 | 17 u |
| H₂S | 2 × 1 + 1 × 32 | 34 u |
| SO₂ | 1 × 32 + 2 × 16 | 64 u |
| HCl | 1 × 1 + 1 × 35.5 | 36.5 u |
| O₂ | 2 × 16 | 32 u |
Oxygen gas is O₂. Two oxygen atoms join to make one molecule. So its mass is 32 u, not 16 u.
One water molecule is 18 u, in a drop or a lake.