You have to know what's what with empirical and molecular formulas, so here goes...
1. The empirical formula gives just the smallest whole number ratio of atoms in a compound.
2. The molecular formula gives the actual numbers of atoms in a molecule.
3. The molecular formula is made up of a whole number of empirical units.
E.g. A molecule has an empirical formula of C4H3O2, and a molecular mass of 166g. Work out its molecular formula.
First find the empirical mass - (4x12) + (3x1) + (2x16) = 48 + 3 + 32 = 83g
But the molecular mass is 166g, so there are 166/83 = 2 empirical units in the molecule.
The molecular formula must be the empirical formula x 2, so the molecular formula = C8H6O4.
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