Formula Unit Mass
Formula unit mass is the sum of the atomic masses of all ions or atoms shown in the formula unit of an ionic compound.
Practice This ConceptMain explanation
Teacher explanation
Ionic compounds do not usually exist as separate molecules. Their formula represents the simplest whole-number ratio of ions in the lattice. So for ionic compounds, we calculate formula unit mass instead of molecular mass.
Example
Formula unit mass of NaCl = 23 + 35.5 = 58.5 u.
Simple analogy
Ionic lattice uses formula unit, not molecule.
Common confusion
Students call NaCl molecular mass. It is better to write formula unit mass because NaCl is an ionic compound with a lattice structure.
Exam tip
First identify whether the compound is ionic. If it is ionic, use the term formula unit mass and calculate from the formula.
Answer writing and exam use
1-mark use
Write the exact meaning of formula unit mass in one clean line.
2-mark use
Define formula unit mass and add one example or condition.
3-mark use
Explain formula unit mass, show the method or example, and mention the common mistake.
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