Relative formula mass is the mass of one formula unit of a compound measured on a scale in which a carbon-12 atom has a mass of exactly 12.

• A. The mass of a molecule
• B. The mass of one formula unit of a compound measured on a scale in which carbon-12 has a mass of exactly 12 units
• C. The mass of a single atom
• D. The mass per mole of a compound

Answer: (B)

Relative formula mass is the mass, in atomic mass units (amu), of one formula unit of a compound measured on a scale where carbon-12 has a mass exactly 12. This means you sum the atomic masses of all atoms in the formula unit and express the result on the C-12 scale. For ionic compounds, we use the term formula unit; for covalent compounds, we often use relative molecular mass, but the idea is the same. The statement captures the defining idea: the mass is a relative, unit-friendly quantity tied to the C-12 standard, not the actual mass in grams per mole, and not just the mass of a single atom.