# What is the unit of molecular weight

## Molar masses and molecular weights - how much does a (macro) molecule weigh?

07.09.2017 Dr. Gerhard Heinzmann, Postnova Analytics GmbH

The molecular weight of a molecule is defined by the sum of the weights of the individual atoms that make up the molecule. The reference point for the weight of the individual atoms is the weight of the carbon isotope 12C; an atomic mass unit is the twelfth part of the mass of the carbon isotope 12C defined.

By definition, the twelfth part has a mole of the carbon isotope 12C a mass of 1 g. The numerical values ​​of the relative molecular mass (dimensionless), the absolute molecular mass (in amu, u or Da; the unit Da is named after the English naturalist John Dalton) and the molar mass of a molecule (in g / mol) are the same.

Mass of the atomic mass unit u: 1 u = 1 Da = 1.660539040 · 10-27kg

The molecular weights of atoms are essentially determined by the number of protons and neutrons present in the atomic nucleus, which have an almost identical mass, whereby the proton carries a positive charge while the neutron carries no charge.

Mass of a proton: 1.6727 x 10-27kg, mass of one neutron: 1.6750 10--27kg

The negatively charged electrons that circle around the atomic nucleus only make a small contribution to the total molecular mass of an atom, as their mass is several orders of magnitude smaller than the mass of the core components of the atom.

### Molecular weights of molecules

Looking at a simple molecule such as b. a water molecule (H.2O) then the molecular weight of this molecule is made up of the weight of the two hydrogen atoms, which each consist of a proton and an electron, and that of the oxygen atom, which consists of 8 protons, 8 neutrons and 8 electrons. The total mass of a water molecule is therefore 18.015 u.

Since the molecular weight of a molecule is a very small value, the mass of a single molecule is multiplied by the Avogadro number (6.022 x 1023), d. H. the weight is calculated to be 6.022 x 1023 Particle. This amount is known as the mole. The mass of one mole 12C atoms in the ground state corresponds exactly to 12 g. The molar mass of water molecules (H.2O) corresponds to 18.015 g / mol.