Difference Between Molar Mass and Molecular Mass

Anyone who is studying chemistry needs to know what molar mass and molecular mass are. These two concepts are very important in the field of chemistry and are very useful numbers in many theories and equations.

Molar Mass vs Molecular Mass

The main difference between molar mass and molecular mass is that the mass of a mole of a substance is measured in molar mass, whereas the mass of a molecule of the substance is measured in molecular mass.

Molar Mass and Molecular Mass

Molar mass is the total mass of 6.022 x 1023 molecules (Avogadro number) of a substance. It is also less accurate when compared to molecular mass.

Its standard unit is g/mol. It is calculated by dividing the total mass of a compound by the number of moles.

On the other hand, the entire mass of a substance’s molecules is known as its molecular mass. It is computed by adding all of the masses of the atoms in a molecule by their composition.

Its standard unit is AMU (atomic mass unit). It is more accurate than molar mass and shows more accuracy during calculations.

Comparison Table Between Molar Mass and Molecular Mass

Parameters of ComparisonMolar MassMolecular Mass
MeaningMass of a mole of a substance.Mass of a molecule of a substance.
DefinitionThe total mass of the 6.022 x 1023 molecules included in the compound.It is defined as the total atomic mass of a molecule including its constituents.
AccuracyLess accurate when compared to molecular mass.Shows accuracy in higher-level calculations.
CalculationsIt is calculated by dividing the mass of compound X by the moles of the same compound.It is calculated using adding the mass of each atom while deriving the atomic mass with the help of the periodic table
Unitg/molAmu (atomic mass unit)
UsesUsed to provide measurements to compounds, atoms, or molecules.Used only to determine the mass of molecules.

What is Molar Mass?

It can be defined as the mass of a substance’s mole. It is calculated using grams per mole (g/mol). The number of entities present in a substance (atoms, molecules, etc.) is known as a mole.

A mole of any substance is 6.022 x 1023 molecules. This number is known as the Avogadro’s number.

To calculate the molar mass, you divide the mass by the mole present.

Molar mass= Mass/ Mole g/mol

Because the number of moles cannot be directly measured, the molar mass of a substance serves as a link between the number of moles and its mass.

For example, a mole of oxygen with a mass of 15.9994 grams means that the molar mass would be 15.9994 g/mol.

Each element’s atomic mass, expressed in g/mol, is its characteristic molar mass. The atomic mass in ‘amu’ can also be computed by multiplying the molar mass constant by the atomic mass in ‘amu’.

If you want to calculate the molar mass of a compound consisting of multiple components, sum the atomic masses of all the constituent atoms.

For example, in NaCl, you will need the atomic mass of its components, namely, sodium and chlorine which are 22.99 g/mol and 35.35 g/mol, respectively. Then by combining them, you’ll get the molar mass which will be 58.44 g/mol.

What is Molecular Mass?

Molecular mass is the mass of a substance that is based on the atomic weight of carbon-12. It can also be defined as the mass of one molecule.

Its unit is amu which is short for atomic mass units. It is also known as “molecular weight”. Molecular mass is also relative to a carbon atom.

A molecule is a composition of similar or different types of atoms that have been arranged in ratios. The molecular mass may differ for the same compound because of the substance’s isotopes.

In practice, it is computed by adding the atomic weights of the atoms that make up the molecular formula of the material. So, to calculate the molecular mass of a compound X, combine the atomic masses of the constituent elements by the formulae.

For example, to determine the molecular mass of H2o, you add the atomic mass of 2 hydrogen atoms and one oxygen atom, which are 2(1.0079) and 15.9994, respectively. The molecular mass obtained is 18.0152 amu.

Another example can be the molecular mass of methane, i.e., CH4. You will have to add the atomic masses of carbon and hydrogen according to their composition in the formula.

So, you will have 12.011 for carbon and 4 x 1 for hydrogen. The molecular mass of CH4 will be 16.

Main Differences Between Molar Mass and Molecular Mass

  1. These two concepts are different from one another. Molar mass refers to the mass of a substance’s mole, whereas molecular mass indicates the mass of the substance’s molecule.
  2. The definition of molar mass is that it is the mass of the Avogadro’s number of a substance. On the other hand, the molecular mass is defined as the sum of the atomic masses of the atoms present in a substance’s molecule based on their composition in the formula.
  3. In terms of accuracy, the molar mass is less accurate than molecular mass, and molecular mass shows accuracy even in high-level calculations.
  4. The S.I. unit of molar mass is g/mol, whereas the S.I. unit of molecular mass is amu.
  5. The molar mass is obtained by dividing the total mass of the moles by the number of moles in the compound. The molecular mass can be calculated by adding all the atomic mass of every individual atom according to their composition in the compound’s formula.
  6. Molar mass can be used to measure compounds, molecules, or atoms. Molecular mass can be used to determine the mass of molecules.


Both molar mass and molecular mass are important topics in the world of chemistry. The mass of a mole of a substance is referred to as molar mass, while the mass of its molecule is referred to as molecular mass.

Their SI units differ as well. Molar mass is measured in g/mol, while molecular mass is measured in amu, which stands for an atomic mass unit. 

Both are widely used topics in chemistry, so it’s advisable to be thorough with them and their differences.


  1. https://www.worldscientific.com/doi/abs/10.1142/S2529732521400010
  2. https://www.sciencedirect.com/science/article/abs/pii/S000862159600273X
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