Empirical formula

A ratio formula ( Empirical IUPAC formula by which no longer valid DIN standard 32641 elementary formula, sometimes also called substance formula ) is the smallest possible number ratios of atoms of chemical elements involved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

Historical Significance

Until the 20th century had ratio formulas of great importance, since they were experimentally often easier or more reliable to determine when the molecule or molecular formula. The ratio formula were many cases again the experimental results, while the molecular formulas sometimes based on additional assumptions. Due to the very precise modern structure determination methods, is just for large but well-defined samples of the case sometimes vice versa. For proteins, for example, it is useful to determine the structure of the molecular formula, from which the ratio formula can be accurately derived. This is often more accurate than by a simple elemental analysis.

The significance of a molecular formula is much higher. In particular, from the molecular formula slightly the ratio formula can be derived, while this only with the help of additional information is vice versa. Ratio formulas therefore have only a relatively small practical importance for molecules. For ionic compounds, however, often only an indication of the ratio formula is useful.

Representation

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

For example, the compound of copper ( I) sulphide is represented by the empirical formula Cu2S. It contains copper and sulfur ions in a 2:1 ratio.

The ratio formula differs in this case from the empirical formula ( molecular formula ) that indicates the actual number of atoms. So would the connection phosphorus pentoxide with the molecular formula P4O10 the ratio formula P2O5. Acetic acid has the empirical formula CH2O but the molecular formula C2H4O2. For water, the ratio formula H2O is equal to the sum formula.

Determination

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