What does the formula unit mass of a substance represent?
Sum of atomic masses of all atoms in a formula unit
The formula unit mass of an ionic (or covalent) substance is defined as the sum of the atomic masses of all the atoms that appear in one formula unit of that substance. For example, the formula unit mass of sodium chloride, NaCl, is the atomic mass of Na (23 u) plus the atomic mass of Cl (35.5 u), giving 58.5 u.
It is not the ratio of atomic masses of metal and nonmetal atoms, nor the difference between molecular and atomic masses, nor the product of atomic numbers. The idea is simply to add up the atomic masses of every atom that appears in the chemical formula, counting each atom the correct number of times.
Hence, the answer is Sum of atomic masses of all atoms in a formula unit.
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