The mass of a single molecule expressed in atomic mass units is called?

• A. Molecular mass
• B. Molar mass
• C. Atomic weight
• D. Formula mass

Answer: (A)

Mass of a single molecule expressed in atomic mass units is called molecular mass. The atomic mass unit is defined so that carbon-12 has a mass of exactly 12 amu, so the molecular mass is simply the sum of the atomic masses of all atoms in the molecule, measured in amu. This value is sometimes called the relative molecular mass. It differs from molar mass, which is the mass per amount of substance and has units of grams per mole, even though the numbers are often numerically similar. Atomic weight is the average mass of atoms of an element (accounting for isotopes), not the mass of a single molecule. Formula mass refers to the mass of a formula unit in ionic substances, expressed in amu, but for a single molecule the correct term is molecular mass.