Ratio formula

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A ratio formula (according to IUPAC empirical formula , according to the no longer valid DIN standard 32641 elementary formula , sometimes also called substance formula ) specifies the smallest possible number ratios of the atoms of the chemical elements involved in a chemical compound . The ratio formula does not take into account the chemical structure of a compound and is the result of a quantitative elemental analysis . It usually differs from the molecular or molecular formula of a compound, in which the real numbers of atoms in the molecule are given.

Historical meaning

Ratio formulas were still very important well into the 20th century, as they were often easier or more reliable to determine experimentally than the molecular or sum formula . In many cases, the ratio formula reflected the experimental results, while the molecular formulas were sometimes based on additional assumptions. Due to the very precise modern structure determination methods, especially for large but well-defined samples, the case is sometimes the other way around. In the case of proteins, for example, it makes sense to use the structure to determine the molecular formula from which the ratio formula can be derived exactly. This is often more accurate than a simple elemental analysis.

The significance of a molecular formula is much higher than that of a ratio formula. In particular, the ratio formula can easily be derived from the molecular formula, while the reverse is only possible with the aid of additional information. Ratio formulas are therefore of relatively little practical importance for molecules. For ionic compounds, on the other hand, it is often only useful to state the ratio formula.


Comparison of different formulas with different degrees of abstraction.
Structural formulas Other modes of representation
Electron formula Valence stroke formula Wedge formula Skeletal formula Constitutional formula Molecular formula Ratio formula
methane Methane electron formula Methane Lewis.svg Methane structure does not exist CH 4 CH 4 CH 4
propane Propane electron formula Propane Lewis.svg Propane structure Propane skeletal formula CH 3 -CH 2 -CH 3 C 3 H 8 C 3 H 8
acetic acid Acetic acid electron formula Acetic acid valence line formula Acetic acid structure Acetic acid skeletal formula CH 3 -COOH C 2 H 4 O 2 CH 2 O
water Water electron formula.svg WasserValenz.svg WasserKonstitution.svg does not exist does not exist H 2 O H 2 O

Representation / explanation

The atoms of an element are identified in the ratio formula by the symbols given in the periodic table (PSE). The respective ratio is indicated by subscript numbers (indices), whereby a 1 is always omitted.

Example: The compound copper (I) sulfide is given by the ratio formula Cu 2 S. It contains copper and sulfur ions in a ratio of 2: 1.

The ratio formula differs in some cases from the empirical formula ( molecular formula ), which gives the actual number of atoms. The compound phosphorus pentoxide with the molecular formula P 4 O 10 would have the ratio formula P 2 O 5 . Acetic acid has the ratio formula CH 2 O, but the empirical formula C 2 H 4 O 2 . For water, the ratio formula H 2 O is the same as the sum formula.


In the case of unknown chemical compounds, the ratio formulas are determined by means of elemental analysis. The qualitative elemental analysis serves to determine the elements present ( detection reactions) and the quantitative elemental analysis to determine the ratio of the atomic numbers. The molecular formula can be determined by determining the molar mass .

Individual evidence

  1. Entry on empirical formula . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.E02063 Version: 2.3.1.
  2. Wolfgang Liebscher, Nomenclature of Inorganic Chemistry , John Wiley & Sons, 2009.