#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..
