Difference between Molecular mass and Molar mass

Published on December 29, 2016

One of the most basic problems in chemistry is explaining what goes on when chemicals react with each other. Since molecules are so tiny, it is hard to talk about them in terms of things we can observe with the naked eye. Trying to convey the molecular mass, or weight, of minuscule objects is just one of the problems scientists run into when talking about chemical reactions. To convey the molecular mass in a way that can be more easily understood, the scientific field adopted the term “mole”. The concept of the mole was devised as a convenient way to simplify calculations in any chemical reaction. In the molar scale, we can talk about amounts that are easier to grasp. So, how is the molecular mass different from the molar mass? Read the definitions below and discover their differences.


Molecular mass vs Molar mass
1 molecule of water (H2O)

Molecular mass refers to the mass of a molecule. It is the total mass of each component atom in the molecule. It is expressed in unified atomic mass units (u) or daltons (Da). 1 Da is equivalent to 1 u.

The atomic mass unit of a molecule is the sum of the masses of its components – the protons, neutrons and electrons. One u is defined as 1/12 (one twelfth) the atomic mass of carbon-12, an atom that has 6 neutrons, 6 protons and 6 electrons. Since the mass of the electrons is very small, the proton and the neutron make up most of the mass of the atom. Each proton or neutron is approximately equal to 1 dalton.

In nature, elements often exist in different variants. Some atoms will have one or more extra neutrons. These are called isotopes of the element and have different atomic masses. In chemical calculations, the atomic mass listed in the periodic table refers to the average mass of all the existing isotopes of that element. It is often called standard atomic mass.

From the molecular formula (e.g. H2O), the molecular mass is calculated by multiplying the standard atomic masses of each component element (e.g. H = 1.008, O = 15.999) by the number of atoms of that element (e.g. H=2, O=1) then adding them all up (e.g. H 1.008 x 2 + O 15.999 x 1 = 18.015).

Molar mass
1 mole of water (approx. 18 grams)

Molar mass refers to the mass of one mole of a substance or compound. A mole is defined as 6.022 x 1023 atoms or molecules of a particular substance. The molar mass is expressed in grams per mole (g/mol). It is numerically equivalent to the standard molecular mass which is expressed in daltons.

The concept of molar mass is very useful in describing amounts and proportions of reactants and products of chemical processes. For example, if a chemical reaction has 1 mole of hydrogen, we know that there are 6.022 x 1023 atoms of hydrogen. If we know the molar mass and the number of moles of hydrogen, we can then calculate the molecular mass of hydrogen in the reaction. The molar mass gives us a simplified way to talk about the masses of compounds or substances in a chemical reaction.

Molecular Mass vs Molar Mass

What is the difference between molecular mass and molar mass? They describe similar characteristics of a compound, but there are a few key differences between the two. The molecular mass is the mass of a single molecule, while the molar mass is the mass of one mole of a substance, or 6.022 x 1023 units of a substance. Units used when computing for molecular mass are either atomic mass unit (u) or daltons (Da). Molar mass, on the other hand, is expressed in grams per mole (g/mol). Molecular mass is measured on a microscopic scale, whereas molar mass is macroscopic.

Comparison Chart

Molecular massMolar mass
Mass of a single moleculeMass of one mole of a substance (6.022 x 1023 molecules)
Expressed in atomic mass unit (u) or daltons (Da)Expressed in grams per mole (g/mol)
Microscopic scaleMacroscopic scale