# How do you get the Molecular Formula from Percent Composition?

Mar 19, 2016

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 ${C}_{5} {H}_{5} N$ or $C {H}_{4}$, the empirical formula is the SAME as the molecular formula.

For larger organic molecules, say sugar, ${C}_{6} {H}_{12} {O}_{6}$, the empirical formula I would get was $C {H}_{2} O$ (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.