Empirical Formulas and Molecular Formulas

  An empirical formula gives the simplest whole number ratio of atoms in a chemical compound. In contrast, a molecular formula specifies the number of atoms in a molecule. Two or more substances may have the same empirical formula but different molecular formulas. For example, CH₂O is both the empirical and the molecular formula for formaldehyde; it is also the empirical formula for such diverse substances as acetic acid, C₂H₄O₂; glyceraldehyde, C₃H₆O₃; and glucose, C₆H₁₂O₆, as well as more than 50 other substances containing 6 or fewer carbon atoms. We may calculate the empirical formula of a compound from its percent composition. To determine the molecular formula, we must know the molar mass of the compound.

  A structural formula provides additional information. For example, the chemically different ethanol and dimethyl ether share the same molecular formula C₂H₆O. Their structural formulas, C₂H₅OH and CH₃OCH₃, reveal structural differences between these compounds that are not shown in their common molecular formula.