Relative formula mass is used for which type of compound?

• A. Ionic compounds
• B. Molecular compounds
• C. Gaseous compounds
• D. Metallic compounds

Answer: (A)

Relative formula mass is used when a substance is made up of ions in a lattice rather than discrete molecules. In ionic compounds, the formula represents the ratio of different ions, so we add together the relative atomic masses of all the atoms in the formula unit to get the mass. For example, in sodium chloride the formula unit is NaCl, so the relative formula mass is 23 (Na) + 35.5 (Cl) = 58.5. This mass corresponds to the formula unit of the compound, not a single molecule.

In contrast, molecular compounds are described by their relative molecular mass, because they exist as actual molecules with a defined number of atoms in each molecule (like H2O or CO2). Metals form a lattice of atoms rather than ions in discrete units, so the same concept isn’t used in the same way for pure metallic substances.