Question

Define molecular mass of a substance.

Answer 1

Definition:

• The molecular mass of a substance is the relative mass of its molecule as compared to the mass of the Carbon-12 atom taken as 12-units.
• It is also defined as the sum of the atomic mass of all the atoms present in a molecule.
• Generally, it is expressed in atomic mass units $\left(\mathrm{amu}\right)$.

Example: Calculating the molecular mass of ${\mathrm{H}}_2\mathrm{O}$

1. As we know, the atomic mass of $\mathrm{H}$ = 1u and $\mathrm{O}$ = 16u
2. Molecular mass of ${\mathrm{H}}_2\mathrm{O}$ = $(2\times 1\mathrm{u})+16\mathrm{u}=18\mathrm{u}$