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Empirical Formula

The empirical formula is the simplest whole number ratio of atoms of each element in a compound.

Some substances do not exist as individual molecules. For example sodium chloride is an ionic salt that exists as a giant lattice. The formula NaCl gives the whole number ratio of atoms of sodium and chlorine.NaCl is also called the formula unit when calculating relative masses.The empirical formula can be calculated using chemical analysis to determine the mass of each element in a compound or the percentage composition by mass of each element in a compound. This can be used to find the number or relative number of moles of each element by dividing the mass or percentage mass by the relative atomic mass. This can then be converted into a whole number ratio in its simplest terms.So to find the number of units of the empirical formula in the molecular formula, divide the relative molecular mass by the relative mass of the empirical formula.

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Specification: AQA Chemistry (7404), AS-level, 3.1.2.4 Empirical and molecular formula. Topic: Amount of Substance.

Key terms in this lesson

Relative atomic mass: The weighted mean mass of an atom of an element compared to 1/12th the mass of a carbon-12 atom.
Relative molecular mass: The weighted mean mass of a molecule compared to 1/12th the mass of a carbon-12 atom.
Mole: The amount of substance that contains as many particles as there are atoms in exactly 12g of carbon-12.
Molecular formula: The actual number of atoms of each element in a molecule.

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