By definition, the empirical formula is the SIMPLEST whole number ratio that defines constituent atoms in a species. The ratio of atoms in that species is 1:2:1 with respect to carbon, hydrogen, and oxygen.
The molecular formula is ALWAYS a mulitple of the empirical formula. Here
Microanalysis can tell you percentage compositions of a compound by mass, and we can use this percentage composition to give an empirical formula. We need an idea of molecular mass before we can calculate the molecular formula of an unknown.