relative molecular mass: meaning, definition, pronunciation and examples

In many practical contexts, they are used interchangeably. However, strictly speaking, 'relative molecular mass' (Mᵣ) is the correct dimensionless term, while 'molecular weight' historically implied a weight and sometimes carries units. In modern usage, the distinction is often blurred.

Because carbon-12 is the standard reference. One twelfth of the mass of a carbon-12 atom is defined as one atomic mass unit (amu) or dalton (Da). Therefore, the relative molecular mass of carbon-12 itself is exactly 12 by definition.

You sum the relative atomic masses (from the periodic table) of all the atoms present in the molecular formula. For example, for water (H₂O): (2 × 1.008) + 15.999 = 18.015.

For ionic compounds like NaCl, the term 'formula mass' or 'relative formula mass' is more accurate, as they do not exist as discrete molecules. The calculation method is identical: sum the atomic masses of all atoms in the formula unit.

The ratio of the average mass of one molecule of a substance to one twelfth of the mass of an atom of carbon-12.

Relative molecular mass is usually technical, scientific, academic in register.

Relative molecular mass: in British English it is pronounced /ˈrɛl.ə.tɪv məˈlɛk.jə.lə mæs/, and in American English it is pronounced /ˈrɛl.ə.t̬ɪv məˈlɛk.jə.lɚ mæs/. Tap the audio buttons above to hear it.