Molecular mass

Written by J.A Dobado | Last Updated on April 22, 2024

What is molecular mass?

The molecular mass (m) (relative molecular mass or formula weight) is the mass of a given molecule in a chemical compound. This mass is measured in units of atomic mass (u) or also called Dalton (Da).

Because different molecules of the same compound can have different isotopes of an element, the molecular mass of two molecules can be different.

Molecular mass should not be confused with molar mass. Although their absolute value is the same, they are expressed in different units of measurement. Molar mass is expressed in g/mol, and is the most appropriate figure when macroscopic quantities of a substance are used.
Molecular mass refers to a single molecule, whereas molar mass corresponds to one mole (NA = 6.022×1023) of molecules.

The definition of molecular weight is synonymous with molecular mass; however, in common practice, it is also highly variable, as are the units used in conjunction with it.
Although molecular masses can be found expressed in g/mol and effectively define it as synonymous with molar mass, molecular mass must be expressed in Da or u.

In molecular biology, the weight of macromolecules is called molecular weight and is expressed in kDa, although the numerical value is usually approximate and representative of an average of the atomic weights of the different atomic isotopes.

The terms molecular mass, molecular weight and molar mass are often used interchangeably in areas of science where no additional precision is needed.

In other areas of science, such a distinction is crucial. Molecular mass is most commonly used when referring to the mass of a single or specific well-defined molecule and less often than molecular weight when referring to a weighted average of a sample.

Prior to the 2019 redefinition of the base units of the international system (SI), quantities expressed in daltons (Da or u) were, by definition, numerically equivalent to identical quantities expressed in g/mol units and were therefore strictly numerically interchangeable.
However, after such redefinition this relationship is only nearly equivalent.

The molecular mass of small to medium-sized molecules, measured by mass spectrometry (MS), can be used to determine the composition of elements in the molecule.

The molecular masses of macromolecules, such as proteins, can also be determined by MS. In addition, methods based on viscosity and light scattering are used to determine molecular mass when neither crystallographic (X-ray) nor MS data are available.

How to calculate molecular mass

The molecular mass can be expressed as follows:

m = Mi x ni

ni is the number of atoms of the ith element in the molecule.
Mi is the atomic mass of the ith element in the molecule.

Similar to atomic mass, molecular mass is expressed in units of atomic mass: umas (u) or daltons (Da), which are equivalent.

Umas are a unit used in chemistry and physics, while daltons are used in biochemistry, as they have the advantage of being able to be used for larger molecules by accepting a multiple, the kilodalton: kDa.

Molar mass calculator


To calculate the molecular mass of a chemical compound, it is sufficient to add the atomic masses of the elements that form the molecule, each multiplied by the number of times that element appears in the formula of the compound.

where Mi = the atomic mass of each element, and ni = the subscript of that element in the chemical formula of the molecule.

For example, for methane of formula CH4, it is calculated as follows:

Relative atomic mass of hydrogen (1H): 1.00797 u.

Relative atomic mass of carbon (12C): 12.01078 u.

C x 1 + H x 4 =

12.01078 x 1 + 1.00797 x 4 = 16.04266 u

This is approximately 16 u.