What is the molecular mass of substance and how it is calculated ?


The average mass of a molecule of a substance is termed as its molecular mass (M).” the molecular mass has the unit of mass, viz., gram, kilogram, atomic mass unit, etc. The accepted unit of molecular mass is atomic mass units (u). Thus, the average mass of a molecule of a substance in atomic mass unit is called its molecular mass.

From the above relationship, one can write

Average mass of a molecule of a substance


= Relative molecular mass of the substance × (Mass of an atom of 126 C in u)/ 12

Or Molecular mass of the substance (M) =Mr × 1u= Mr, u


Molecules are composed of two or more atoms of the same or different elements. Mass is an additive property. So, the molecular mass of any substance can be obtained by adding together the atomic masses of all the atoms present in one molecule of the substance. For example, the molecular formula of water is H2O. Therefore,

Molecular mass of H2O = (2 × atomic mass of hydrogen) + (1× atomic mass of oxygen)


We know that,

Atomic mass of hydrogen = 1u

And Atomic mass of oxygen = 16 u

Therefore, Molecular mass of H2O = (2× 1 u) + (1× 16u)= (2u +16u)=18u

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