Percentage Composition
% composition gives the percentage of each element in a compound by mass.
For instance Consider the formula copper (II) oxide CuO. The molar mass of the compound is
63.55 g/mol + 16.00 g/mol = 79.55 g/mol
If I have one mole of the compound, that would be equivalent to a mass of 79.55 g
(I chose one mole for convenience)
The percentage of copper in the sample would be 63.55 g / 79.55 g x 100 = 79.9%
The percentage of oxygen in the sample would be 100 - 79.9 = 20.1 %
It is possible to work backwards to find the empirical formula of a compound from % by mass
information.
The following example was taken from the Journal of Chemical Education;
Determination of an empirical formula using experimental data
Magnesium burns in air to form a compound containing magnesium and oxygen atoms. The
following data was collected in an experiment to determine the formula of the compound that is
formed when magnesium is burned in air.
