Molecular mass represents

• A. The total mass of all elements in the compound
• B. The mass of one mole of the compound
• C. The fraction of mass contributed by each element
• D. The number of atoms in the empirical formula

Answer: (B)

Molecular mass is found by summing the atomic masses of all atoms in a molecule, giving the mass per molecule in atomic mass units. That same number also tells you the mass per amount of substance when you express it in grams per mole, because one mole contains Avogadro’s number of molecules. In other words, the molecular mass you calculate for a molecule is numerically equal to the molar mass, just with different units.

For example, water has a molecular mass of about 18.015 amu per molecule, which corresponds to a molar mass of about 18.015 g per mole. That connection is why the best description is that molecular mass represents the mass of one mole of the compound when you use g/mol as the unit.