To consider the masses of molecules measured in grams, for example, would be to deal inconveniently with extremely small numbers. Rather, the mass of a molecule is compared with that of an atom of carbon-12. The relative atomic mass of carbon-12 is taken to be 12. Relative masses have no units because they have cancelled in their calculation. A relative molecular mass can be calculated easily by adding together the relative atomic masses of the constituent atoms. For example, ethanol, CH3CH2OH, has a Mr of 46.
Some care is needed. The term 'relative molecular mass' is sometimes used to refer to ionic compounds such as sodium chloride, NaCl. Such compounds are not molecular, and more accurately the term 'relative formula mass' can be used.
The mass spectrometer is also used to measure relative molecular masses (see relative atomic mass). The molecular ions formed in the instrument can often fragment, and it is from the the relative masses and abundances of these fragments that information about molecular structure can be deduced.
See also The Mole | Relative Atomic Mass.