Molecular mass is defined as the mass of one molecule of an element or a compound relative to 1/12 of the mass of a carbon-12 atom.

• A. The mass of one mole of molecules
• B. The mass of one molecule of an element or a compound relative to 1/12 of the mass of a carbon-12 atom
• C. The mass of a single atom
• D. The mass of a formula unit

Answer: (B)

Molecular mass is a comparison of how heavy a molecule is relative to a standard, namely 1/12 of the mass of a carbon-12 atom. It’s found by adding up the atomic masses of all the atoms in the molecule (in atomic mass units). This makes molecular mass a dimensionless quantity in u, and its numerical value equals the molar mass in g per mole. For example, water has two hydrogens (2 × 1) and one oxygen (16), giving a molecular mass of 18, so its molar mass is 18 g/mol. The other statements mix up concepts: the mass of one mole (molar mass) is a different quantity with units of g/mol; the mass of a single atom refers to atomic mass, not the whole molecule; and a formula unit is used for ionic compounds, not molecules.