Chemistry 21 – Honeycutt Science

Formulas of Compounds

This topic distinguishes between an empirical formula and a molecular formula. Both are used to describe compounds. They are similar in several respects, but they are different from each other. Generally, they serve different purposes. This topic emphasizes empirical formulas. Even so, it is instructive to understand what a molecular formula is – partially for the purpose of contrast. A molecular formula provides the composition of the molecules that are present, whereas an empirical formula expresses the smallest whole-number ratio of atoms present. An example may provide the clearest difference between the two types of formulas.

Empirical formula: There are three molecules which have identical empirical formulas. The three molecules in this example are formaldehyde, erythrose, and glucose. Each of these have the same empirical formula which is CH₂O. This is the smallest whole-number ratio of atoms present in each of these three compounds. All three molecules have the same ratio of carbon, hydrogen, and oxygen atoms. For every one carbon atom, there are two hydrogen atoms and one oxygen atom.
Molecular formula: In contrast, the molecular formulas of these three molecules are formaldehyde (CH₂O), erythrose (C₄H₈O₄), and glucose (C₆H₁₂O₆). Notice that the molecular formula describes the total number of atoms present (the composition) within their respective molecules.