underbrace"Benzene"_"Molecular formula"-=C_6H_6
The "empirical formula" is the simplest whole number ratio defining constituent atoms in a species...and thus for benzene, whose "molecular formula" is C_6H_6...the "empirical formula" is simply CH...
Typically, we interrogate the empirical formula by experimental means (and that is what "empirical" means, "by experiment"), usually combustion analysis, to give the percentage by mass of, carbon, hydrogen, and nitrogen, and by a Mohr titration to give halide content. And then some means is devised to measure the MOLECULAR MASS of the species...for benzene of course this is 78*g*mol^-1...and thus with the empirical formula...we calculate that (CH)_n=78*g*mol^-1
(12.011*g*mol^-1+1.00794*g*mol^-1)_n=78*g*mol^-1
And thus n=6...and the molecular formula is C_6H_6.
While this is fairly trivial in this instance, for more complex organic and inorganic molecules, some means must be employed to measure the molecular/formula mass of the molecule... And this is not always straightforward..
