Relative atomic mass
The relative atomic mass of an elementA substance made of one type of atom only. shows its massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). compared with the mass of atoms of other elements. The relative atomic mass of carbon is 12, while the relative atomic mass of magnesium is 24. This means that each magnesium atom is twice the mass of a carbon atom.
Relative formula mass
Relative atomic masses can be used to find the relative formula massThe relative formula mass (Mr) of a compound is calculated by adding together the relative atomic masses (Ar) of the atoms present in the compound. of a compound.
To find the relative formula mass (Mr) of a compound, you add together the relative atomic mass values (Ar values) for all the atoms in its formula.
Question
Find the Mr of carbon monoxide, CO.
The Ar of carbon, C = 12, and the Ar of oxygen, O = 16.
The Mr of carbon monoxide, CO = 12 + 16 = 28.
Question
Find the Mr of sodium oxide, Na2O.
The Ar of sodium, Na = 23, and the Ar of oxygen, O = 16.
The Mr of sodium oxide, Na2O = (23 × 2) + 16 = 62.
(Note, the workings could also be shown as 23 + 23 + 16)
The relative formula mass of a substance, shown in grams, is called one mole of that substance. So one mole of carbon monoxide has a mass of 28 g, and one mole of sodium oxide has a mass of 62 g.
Relative formula mass calculations
The table shows some more examples of relative formula mass calculations, using the relative atomic mass values listed beneath it.
| Compound | Formula | Calculation | Relative formula mass |
| Water | H2O | 1 + 1 + 16 = | 18 |
| Sodium hydroxide | NaOH | 23 + 16 + 1 = | 40 |
| Magnesium hydroxide | Mg(OH)2 | 24 + 16 + 16 + 1 + 1 = (remember that there are two of each atom inside the brackets) | 58 |
| Compound | Water |
|---|---|
| Formula | H2O |
| Calculation | 1 + 1 + 16 = |
| Relative formula mass | 18 |
| Compound | Sodium hydroxide |
|---|---|
| Formula | NaOH |
| Calculation | 23 + 16 + 1 = |
| Relative formula mass | 40 |
| Compound | Magnesium hydroxide |
|---|---|
| Formula | Mg(OH)2 |
| Calculation | 24 + 16 + 16 + 1 + 1 = (remember that there are two of each atom inside the brackets) |
| Relative formula mass | 58 |
- Ar of hydrogen, H = 1
- Ar of oxygen, O = 16
- Ar of sodium, Na = 23
- Ar of magnesium, Mg = 24
