Concept:A formula unit is the simplest ratio of ions in an ionic compound, and its mass is the sum of the atomic masses of all atoms in that unit.
Explanation:Ionic compounds like
NaCl do not form discrete molecules; they exist as crystal lattices.
Hence, the term "formula unit" is used instead of "molecule".
The formula unit mass is calculated by adding the atomic masses of all atoms in the empirical formula of the ionic compound.
For example, in
NaCl, atomic mass of Na is
23 u and Cl is
35.5 u, so the formula unit mass is
23+35.5=58.5 u.
Option A ("Mass of an atom") refers to atomic mass, not formula unit mass.
Option B ("Mass of one molecule") applies to covalent compounds, not ionic compounds.
Option D ("Mass of one mole of a compound") is the molar mass, which is different from formula unit mass.
Only Option C correctly defines formula unit mass as the sum of atomic masses of all atoms in the formula unit of an ionic compound.
Answer:C. Sum of atomic masses of all atoms in the formula unit of an ionic compound.