Molecular mass

As molecular mass and molecular mass (English molecular mass ), formerly molecular weight (English molecular weight), refers to the sum of the atomic masses of all atoms in a molecule. When one speaks of salts formula mass, as salts are composed of ions. It is between relative molecular mass ( has no unit) and absolute molecular weight ( in kg, g or u or Da) differed.

The molecular weight, however, is the sum of atomic weights of the elements of a molecule based on one mol of said particles and in mass per mole of ( usually g / mol) specified. The numerical value of the molecular weight and the molar mass is equal.

Relative molecular mass

The relative molecular mass is based on one twelfth of the mass of the carbon isotope 12C normalized molecular mass and thus has no unit of measurement:

Example

Calculate the relative molecular mass of water and carbon dioxide from the individual mean relative atomic masses:

Absolute molecular mass

The absolute molecular mass mM is obtained by dividing the molar mass M of a molecule by the number of particles in one mole of NA:

Thus, the absolute molecular mass of water ( H2O) is given by:

Since specifying g gives absolute molecular mass in the unit impractical a small value, the atomic mass unit is mostly u, amu or Da used. An atomic mass unit is defined as a twelfth of the mass of carbon isotope 12C. By definition, the twelfth part of one mole of carbon isotope 12C has a mass of 1 g Thus, the numerical values ​​of, relative molecular mass ( dimensionless), absolute molecular mass ( in u or Da) and molar mass of a molecule is equal to ( in g / mol).

Determination

Determining the molecular mass u can in principle according to the same method as for the determination of the molar mass. Since the numerical values ​​of both are the same, the molecular mass follows directly from the experimentally determined molar mass. The historical method such as the method according to Bunsen, the method of Dumas and Victor Meyer, the cryoscopic and ebullioscopy are at best even for the training or teaching chemistry is important. Today the molecular weight is determined substantially over the mass.