Empirical formula

Use in chemistry

In chemistry, the empirical formula of a chemical compound is a simple expression of the relative number of each type of atom in it. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. Empirical formulae are the standard for most ionic compounds, such as CaCl₂, and for macromolecules, such as SiO₂. The term empirical refers to the process of elemental analysis, a technique of analytical chemistry used to determine the relative percent composition of a pure chemical substance by element.

In contrast, the molecular formula identifies a multiple of the smallest whole number ratio in moles.

For example, n-hexane, a chemical compound has the molecular formula CH₃CH₂CH₂CH₂CH₂CH₃, implying that it has a straight chain structure, 6 carbon atoms, and 14 hydrogen atoms. Hexane's molecular formula is C₆H₁₄, and its empirical formula would be C₃H₇ showing a C:H ratio of 3:7.

Examples of common substances

Substance	Molecular formula	Empirical formula
Water	H2O	H2O
Methane	CH4	CH4
Benzene	C6H6	CH
Sulfur	S8	S

Use in physics

In physics, an empirical formula is a special mathematical equation that predicts observed results, but has no known theoretical basis to explain why it works, see also Phenomenology.

An example was the Rydberg formula to predict the wavelengths of hydrogen spectral lines. Proposed in 1888, it perfectly predicted the wavelengths of the Lyman series, but it was not clear why it worked until Niels Bohr produced his Bohr model of the atom in 1913.