Question

How do you get the Molecular Formula from Percent Composition?

Answer 1

The simple answer is that you cannot do so. Elemental analysis does not lead to the molecular formula.

Explanation:

Get an organic sample, and combust it in a furnace, you can shunt the carbon dioxide and water gases to a gas chromatograph, and measure these quantities in relation to a standard. Percentage compositions with respect to `C`, and `H`, and `N` result.

We use these percentages to calculate an EMPIRICAL FORMULA, which is the simplest whole number ratio that defines constituent atoms in a species. If we take simple organic (molecular) formula of `C_5H_5N` or `CH_4`, the empirical formula is the SAME as the molecular formula.

For larger organic molecules, say sugar, `C_6H_12O_6`, the empirical formula I would get was `CH_2O` (because this is the smallest ratio). The point is that the molecular formula is always multiple of empirical formula: `"Molecular formula" = ("Empirical formula")_n`. If an estimate of molecular mass is obtained, the empirical data can be used to provide a molecular formula.

There should be many examples on these boards of specific empirical/molecular formula calculations.