How do you determine if a formula is not molecular?

1 Answer
Aug 26, 2017

An empirical formula will always represent the lowest whole number ratios of the elements in the compound.

A molecular formula indicates the actual number of atoms of elements in a molecule of the compound. Sometimes it's the same as the empirical formula, but often it is a multiple of the empirical formula.


For example, the empirical formula for carbon dioxide is #"CO"_2"#, and its molecular formula is also #"CO"_2"#. However, the empirical formula for glucose is #"CH"_2"O"#, and its molecular formula is #"C"_6"H"_12"O"_6"#.

We can determine the molecular formula from an empirical formula by comparing the empirical formula mass (usually calculated) with the molecular formula mass (usually given). If the two masses are the same, then the empirical and molecular formula are the same. If the two masses are different, then the empirical formula is multiplied by a number that will give the correct molecular formula.

This is the case for glucose. The empirical formula mass for glucose is #"30.026 u"#. Its molecular mass is #"180.156 u"#. Divide the molecular mass by the molar mass, which gives #6#. The subscripts in #"CH"_2"O"# must be multiplied by #6# in order to get the correct molecular formula for glucose, which is #"C"_6"H"_12"O"_6"#.