Final answer:
The term 'molecular mass' can be used interchangeably with 'formula mass' for substances with discrete molecules, representing the sum of the atomic masses of atoms within a molecule, and is related to 'molar mass' which is the substance's mass per mole.
Step-by-step explanation:
The term molecular mass is used interchangeably with formula mass by many chemists in referring to substances that contain discrete molecules. The molecular mass of a substance is the sum of the average masses of the atoms in one molecule of that substance, with each mass being multiplied by the respective subscript in the molecular formula. When speaking of molecular mass, we are often using atomic mass units to express these sums. Furthermore, the molar mass is related to molecular mass, since it is the mass in grams per mole of a substance, which corresponds to Avogadro's number of molecules or formula units.
For covalent compounds, the term molecular mass is especially significant because these substances have a precise molecular formula that reflects the actual number of atoms of each element within a molecule. The calculation of molecular mass uses the weighted average atomic mass of the elements involved, which takes the natural abundance of isotopes into consideration.