In chemistry, empirical formula expresses the relative number of atoms of the different elements present in a compound or structure, using the simplest whole number ratio.  The molecular formula, by comparison, expresses the actual number of atoms present in a structure or compound. Thus, for instance, the empirical formula of acetic acid is CH20, whereas its molecular formula is C2H4O2 or twice CH2O.
1. Baynes, Thomas S. (1833). “Chemical Notation”, Encyclopedia Britannica.
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