# Molecular mass

As molecular weight , and molecular mass ( English molecular mass ), formerly molecular weight ( English weight molecular ), referred to the sum of the atomic weights of all atoms in a molecule . In the case of salts , one speaks of formula mass , since salts are made up of ions . A distinction is made between relative molecular mass (has no unit of measurement ) and absolute molecular mass (given in kg , g or u or Da).

The molar mass , on the other hand, is the sum of all atomic masses of the individual elements of a molecule based on one mole of these particles and is given in mass per mole (usually g / mol). The numerical value of the relative molecular mass and the molar mass is the same.

## Relative molecular weight

The relative molecular mass is the molecular mass standardized to one twelfth of the mass of the carbon isotope 12 C and therefore has no unit of measurement:

${\ displaystyle M_ {R} = 12 \, {\ frac {m {\ text {(molecule)}}} {m \ mathrm {(^ {12} C)}}}}$

### example

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

water
Sum formula: ${\ displaystyle \ mathrm {H} _ {2} \ mathrm {O}}$
${\ displaystyle M_ {R} = (2 \ cdot 1) + 16 = 18}$
carbon dioxide
Sum formula: ${\ displaystyle \ mathrm {CO} _ {2}}$
${\ displaystyle M_ {R} = 12 + (2 \ cdot 16) = 44}$

## Absolute molecular mass

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

${\ displaystyle m_ {M} = {\ frac {M} {N _ {\ mathrm {A}}}}}$

The absolute molecular mass of water (H 2 O) is given by:

${\ displaystyle M = 18 {,} 015 \, \ mathrm {g \, mol} ^ {- 1}}$
${\ displaystyle N _ {\ mathrm {A}} = 6 {,} 022 \ cdot 10 ^ {23} \, \ mathrm {mol} ^ {- 1}}$
${\ displaystyle \ Rightarrow m_ {M} = {\ frac {18 {,} 015 \, \ mathrm {g \, mol} ^ {- 1}} {6 {,} 022 \ cdot 10 ^ {23} \, \ mathrm {mol} ^ {- 1}}} = 2 {,} 9916 \ cdot 10 ^ {- 23} \, \ mathrm {g}}$

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

## determination

The determination of the molecular mass in u can in principle be carried out according to the same procedure as for the determination of the molar mass . Since the numerical values ​​are the same for both, the molecular mass results directly from the experimentally determined molar mass. The historical methods, such as the Bunsen method , the Dumas and Victor Meyer method , cryoscopy and ebullioscopy are at best still of importance for education or chemistry lessons . Today, the molecular mass is mainly determined using mass spectrometry .

## Individual evidence

1. Der Brockhaus, Science and Technology , Mannheim; Spectrum Academic Publishing House, Heidelberg, 2003.
2. ^ R. Brdička: Fundamentals of physical chemistry, VEB Deutscher Verlag der Wissenschaften 1967.