formula, in chemistry: Molecular and Empirical Formulas

Molecular and Empirical Formulas

If a compound is molecular, the molecular formula is preferred to the empirical formula since it gives more information. A molecule of glucose, for example, consists of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. Its molecular formula, C₆H₁₂O₆, displays this information explicitly; the empirical formula is CH₂O. From the formula one can also deduce the proportion of the atoms of each element making up the compound: one atom of carbon to every two atoms of hydrogen to every one atom of oxygen (6 : 12 : 6=1 : 2 : 1). The empirical formula of glucose, CH₂O, shows only the proportion, not the actual number of atoms.

Many compounds may have the same empirical formula. For example, formaldehyde, each molecule of which consists of one carbon atom, two hydrogen atoms, and one oxygen atom, has the molecular formula CH₂O, which is identical to the empirical formula of glucose. Another example is furnished by ethyne (acetylene), whose molecular formula is C₂H₂, and benzene, whose molecular formula is C₆H₆. Both have the same empirical formula, CH.

In addition to showing the actual number of atoms, molecular formulas are also more useful than empirical formulas in that they explicitly show radicals. For example, the molecular formula for the compound aluminum sulfate, Al₂(SO₄)₃, shows that it contains three sulfate radicals (SO₄). The empirical formula, Al₂S₃O₁₂, does not show this. When only one radical is present in the molecule, the parentheses and subscript are omitted, e.g., CuSO₄ for cupric sulfate. Other groups are also shown in molecular formulas, e.g., the water molecules in the mineral chalcanthite (blue vitriol), which consists of cupric sulfate atoms to each of which are attached five water molecules. Its molecular formula is CuSO₄·5H₂O, its empirical formula CuSO₉H₁₀.

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