How do you get the Molecular Formula from Percent Composition?

1 Answer
Mar 19, 2016

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


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.