How is the empirical formula used?

The empirical formula is the simplest whole number ratio that defines the constituent atoms in a species. The molecular formula is always a whole number multiple of the empirical formula. In some circumstances the empirical formula and the molecular formula are the same.

Combust a (typically) organic compound in a furnace, and you get carbon dioxide and water (and sometimes nitrogen gas). Feed these gases into a chromatograph, and you get a very accurate measurement of the the percentage by mass of #C#, #H#, #N#, (measurement of #O# is not so straightforward, sometimes percentage of #O# is simply taken as the balance percentage, i.e. #O%=100%-C%-H%-N%#). This can be converted into an empirical formula by standard means, and there are many examples of the process here.

Combustion analysis does not give the molecular formula. This must be determined by some other means: mass spectroscopy; molecular mass determination.

Now the molecular formula is always a multiple of the empirical formula.

i.e. #"Molecular formula"# #=# #"(Empirical formula)"xxn#. Of course this multiple, #n#, may be #1#, in some circumstances.

Knowledge of the empirical formula plus the molecular mass allows us to simply determine this number #n#, and thus knowledge of the molecular formula is provided