Question
Explain: Formula mass with an example.
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Answer
- The formula mass of a substance is the sum of atomic masses of the atoms present in the formula.
- In substances such as sodium chloride, positive (sodium), and negative (chloride) entities are arranged in a three-dimensional structure in a way that one sodium $\left( Na ^{+}\right)$ion is surrounded by six chlorides $\left( Cl ^{-}\right)$ions, all at the same distance from it and vice versa. Thus, sodium chloride does not contain discrete molecules as the constituent units.
- Therefore, NaCl is just the formula that is used to represent sodium chloride though it is not a molecule.
- In such compounds, the formula (i.e., NaCl) is used to calculate the formula mass instead of molecular mass.
$=23.0 u+35.5 u=58.5 u$
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