Chemistry / Unit 3 Chem 0391 worked example, 60 to 90 seconds
Determining Molecular Formula from Empirical Formula and Molar Mass
The idea
Determining a molecular formula from an empirical formula means dividing the compound's known molar mass by its empirical formula's mass, then multiplying every empirical subscript by that whole-number result to expand the ratio back out.
In practice
Nicotine has the empirical formula C5H7N, with an empirical formula mass near 81.13 g/mol, and a separately measured molar mass near 162.26 g/mol. Dividing 162.26 by 81.13 gives almost exactly 2, so every subscript in C5H7N is doubled: five carbons become ten, seven hydrogens become fourteen, and one nitrogen becomes two, giving the molecular formula C10H14N2, twice the empirical formula's mass and twice its atom count.
Where people slip
A division result far from a whole number signals an error upstream, usually in the empirical formula itself, not a reason to round the multiplier to the nearest integer anyway.
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