What is the formula for cesium bromide

A Ratio formula indicates the numerical ratio in which individual types of atoms or ions are present in a chemical compound. On the other hand, a different quantitative ratio can be given in the form of a molecular or empirical formula. The ratio formula describes part of the chemical structure of compounds.

Components of the ratio formula

The types of atoms are denoted in the ratio formula by the symbols given in the PSE. The respective ratio is indicated by subscripts, whereby a 1 is always omitted.

example: the compound copper (I) sulfide is given by the ratio formula Cu2S. It contains copper and sulfur particles (ions) in a ratio of 2: 1.

Ratio and molecular / sum formula

The ratio formula is used to describe all lattice-like substances and always indicates the smallest possible numerical ratio. It differs from that Molecular formula (also Molecular formula called), which gives the actual atomic numbers. So the compound with the molecular formula P4O10 only the relation formula {P2O5}.

Determination and application of the ratio formula

It is mainly used in chemical calculations (stoichiometry), for example to calculate the amount of substance and mass ratios in compounds or in chemical reactions using the reaction scheme.

In the case of unknown chemical compounds, the ratio formulas are determined by means of elemental analysis, whereby the qualitative elemental analysis is used to determine the elements present (detection reactions) and the quantitative elemental analysis is used to determine the ratio formula. Determining the molar mass then leads to the empirical formula of the compound.

molecule Molecular formula Ratio formula Structural formula
benzene C.6H6 {CH}


Ratio formulas were still very important well into the 20th century, as they were often easier or more reliable to determine experimentally than the empirical formula or molecular 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 structure determination methods that are often used today, especially for large but well-defined samples, the case is sometimes the other way around in well-equipped laboratories: for proteins, for example, it makes sense to use the structure to determine the molecular formula from which the ratio formula is derived exactly can - often more precisely than through a simple elemental analysis.

The informative value of a molecular formula is much higher - in particular, one can easily derive the ratio formula from the molecular formula, while the inversion is only possible with the help of additional information. Therefore, ratio formulas are of relatively little practical importance for molecules today. For ionic compounds, on the other hand, it is usually only useful to state the ratio formula.

Introductory examples

Ratio and sum formulas for simple inorganic compounds

  • Hydrogen chloride: HCl
  • Barium chloride: BaCl2
  • Ammonia: NH3
  • Sulfuric acid: H2SO4
  • Nitric acid: ENT3
  • Sodium hydroxide: NaOH
  • Calcium hydroxide: Ca (OH)2
  • Alumina: Al2O3
  • Water: H2O
  • Ammonium sulfide: (NH4)2S.
  • (Di) ammonium carbonate: (NH4)2CO3

Ratio formulas are also used for alloys, for example Ni3Al.

Ratio and sum formulas for cations, complexes and anions

  • Blood liquor salt: K4(Fe (CN)6)
  • Berlin blue: Fe4(Fe (CN)6)3

Molecular formulas for organic compounds

  • Acetic acid: C.2H4O2
  • Dimethylglyoxime: C.2H4N2O2

Category: Chemical quantity