Molecular mass is the sum of the masses of all the atoms in the molecule, denoted as m.
The mass of the atoms is measured in the average atomic mass unit, a weighted average of all the element's naturally occurring isotopes.
The sum is represented in amu (μ) or Da unit.
For example:
The molecular mass (m) of water (H2O) is
= atomic mass of 2H +atomic mass of one O
= (2 x 1.008) + 15.999= 18.00 μ
Since the atomic mass is also known as the atomic weight, the sum of such atoms in the molecular mass is also known as the molecular weight.