The #"empirical formula"# is the simplest whole number ratio defining constituent atoms in a species. Given say a #"molecule"# of #N_2O_4# or #C_2H_6#...the empirical formulae are #NO_2# and #CH_3# by the given definition.
And of course, the #"molecular formula"# is a whole number multiple of the #"empirical formula"#...in general combustion analysis gives us #%C;H;N#...and we can use this to approach the empirical formula. For the molecular formula we need a determination of molecular mass.