Molecular Mass Definition

What Molecular Mass Is and How to Calculate It

Molecular mass is the sum of the atomic masses in a molecule.
Molecular mass is the sum of the atomic masses in a molecule. Lawrence Lawry, Getty Images

Molecular Mass Definition

Molecular mass is a number equal to the sum of the atomic masses of the atoms in a molecule. The molecular mass gives the mass of a molecule relative to that of the 12C atom, which is taken to have a mass of 12. Molecular mass is a dimensionless quantity, but it is given the unit Dalton or atomic mass unit as a means of indicating the mass is relative to 1/12th the mass of a single atom of carbon-12.

Also Known As

Molecular mass is also called molecular weight. Because the mass is relative to carbon-12, it's more correct to call the value "relative molecular mass".

A related term is molar mass, which is the mass of 1 mol of a sample. Molar mass is given in units of grams.

Sample Molecular Mass Calculation

Molecular mass may be calculated by taking the atomic mass of each element present and multiplying it by the number of atoms of that element in the molecular formula. Then, the number of atoms of each element is added together.

For example. to find the molecular mass of methane, CH4, the first step is to look up the atomic masses of carbon C and hydrogen H using a periodic table:

carbon atomic mass = 12.011
hydrogen atomic mass = 1.00794

Because there is no subscript following the C, you know there is only one carbon atom present in methane. The subscript 4 following the H means there are four atoms of hydrogen in the compound.

So, adding up the atomic masses, you get:

methane molecular mass = sum of carbon atomic masses + sum of hydrogen atomic masses

methane molecular mass = 12.011 + (1.00794)(4)

methane atomic mass = 16.043

This value may be reported as a decimal number or as 16.043 Da or 16.043 amu.

Note the number of significant digits in the final value.

The correct answer uses the smallest number of significant digits in the atomic masses, which in this case is the number in the atomic mass of carbon.

The molecular mass of C2H6 is approximately 30 or [(2 x 12) + (6 x 1)]. Therefore the molecule is about 2.5 times as heavy as the 12C atom or about the same mass as the NO atom with a molecular mass of 30 or (14+16).

Problems Calculating Molecular Mass

While it's possible to calculate molecular mass for small molecules, it's problematic for polymers and macromolecules because they are so large and may not have a uniform formula throughout their volume. For proteins and polymers, experimental methods may be used to obtain an average molecular mass. Techniques used for this purpose include crystallography, static light scattering, and viscosity measurements.