# What is absolute mass in chemistry

As Molecular mass, also molecular mass (engl. molecular mass, molecular weight), earlier Molecular weight, is the sum of the atomic masses of all atoms in a molecule. In the case of salts, one speaks of formula mass, since salts are made up of ions, not molecules.

It will be between more relative (has no unit of measure) and absolute molecular weight (given in kg, G or. u or There) differentiated.

### Relative molecular mass

The relative molecular mass is one-twelfth of the mass of the carbon isotope12C standardized molecular mass and therefore has no unit of measurement.

Calculation of the relative molecular mass of water and carbon dioxidefrom the individual relative atomic masses: Formula: H2O Mrel = 2 × 1 + 16 = 18 Formula: CO2 M.rel = 12 + 2 × 16 = 44

### Absolute molecular mass

The absolute molecular mass can be expressed in the unit of measurementG can be specified. However, since the numerical values ​​are very small, the atomic mass unit is preferredu or There is used, which means the twelfth part of the mass of the carbon isotope12C understands. By definition, the twelfth part has a mole of the carbon isotope 12C a mass of 1 g. The absolute molecular mass (in u or There) and the molar mass of a molecule (in g / mol) numerically identical.

The absolute molecular mass mM. is obtained by dividing the molar massM. of a molecule by the number of particles in a moleNA.:

Calculation of the absolute molecular mass of water (H.2O): M = 18.015 g mol-1 NA. = 6,022·1023 mol-1 mM. = 18.015 g mol-1 / 6,022·1023 mol-1 = 2,9916·10-23 G

Another way of determining molecular weight mM. of any molecule is the addition of the atomic masses mA. of all atoms involved in the structure of the molecule. If a chemical element occurs more than once in the molecule in question, the atomic mass of this element has to be taken into account several times (n times) when adding the atomic mass. The atomic masses are given in the periodic table of the elements.