How is the empirical formula used?
For the determination of molecular formula. How?
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
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.
Knowledge of the empirical formula plus the molecular mass allows us to simply determine this number