Which mass term expresses the total mass of a molecule in terms of atomic masses, ignoring charge?

• A. Relative molecular mass
• B. Relative formula mass
• C. Relative atomic mass
• D. Mole

Answer: (A)

To find the mass of a molecule, add up the masses of all the atoms it contains, using atomic mass units. Since charge doesn’t contribute to mass, you ignore the electrical charge when doing this.

Expressing that total as a ratio to 1/12 of carbon-12 gives the relative molecular mass. This term is the one you’d use for a discrete molecule, like H2O, where the total mass is the sum of the atomic masses of the atoms present (for water, 2×1 + 16 = 18).

Relating to other terms: relative atomic mass is the mass of a single atom, not a molecule. Relative formula mass is used for the mass of the formula unit in compounds, often ionic ones, which is similar in idea but applies to formulas rather than whole molecules. A mole, meanwhile, is a quantity of substance, not a mass term.