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Question
Explain formula mass with an example.
Solution
- 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 (Na+) ion is surrounded by six chlorides (Cl–) 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.
e.g. Formula mass of sodium chloride = atomic mass of sodium + atomic mass of chlorine
= 23.0 u + 35.5 u
= 58.5 u
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