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When are the empirical formula and molecular formula the same?

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representations of molecular structure — molecule Several methods of representing a molecule's structure. In Lewis structures, element symbols represent atoms, and dots represent electrons surrounding them. A pair of shared electrons (covalent bond) may also be shown as a single dash. The ball-and-stick model better illustrates the spatial arrangement of the atoms. For aromatic compounds, the Kekulé structure is common, in which each bond is represented by a dash, carbon atoms are implied where two or more lines meet, and hydrogen atoms are usually omitted. Bond-line formulas, similar to the Kekulé structure, are often used for complex nonaromatic organic compounds. Sugars are often drawn as Fischer projections, in which the carbon “backbone” is drawn as a straight vertical line, with carbon atoms implied where horizontal lines intersect the vertical one.

The empirical formula and the molecular formula of a substance are the same when the simplest ratio of elements in a compound is identical to the actual number of atoms of each element present in a single molecule of that substance.12

While an empirical formula provides only the simplest whole-number ratio of atoms, the molecular formula specifies the exact count of every atom in the molecule. In many instances, these two expressions differ; for example, ethylene has an empirical formula of ${CH}_{2}$ but a molecular formula of $C_{2} H_{4}$ . However, for substances like water, where the simplest ratio of atoms is the same as the actual composition of the molecule ( $H_{2} O$ ), the empirical and molecular formulas are identical.312