


Percent Composition of Compounds
There are two different ways to describe the composition
of a compound: in terms of the number of its constituent atoms (like
C_{2}H_{6}) and in terms
of the percentages (by mass) of its elements. When showing the constituent
atoms of a molecule, you can either show the chemical formula, which
shows the real number of atoms in the molecule, like C_{2}H_{6},
or show the empirical formula, which merely shows their
relative amounts in a substance, so the above molecular formula
would be expressed as CH_{3}.
You can describe the composition of a compound in terms
of the weights of its constituent elements by determining the percent
composition of particular elements in the molecule. To calculate
percent compositions, you would find the weight of each constituent atom,
then figure out what percent of the total molecular weight it makes
up. Consider ethanol, C_{2}H_{5}OH.
Taking subscripts into consideration, you have 2 mols of carbon,
6 mols of hydrogen (5 + 1), and 1 mol of O. Now convert moles into
grams for each constituent element as well as for the entire molecule:
Mass of C = 2 12.01= 24.02 g
Mass of H = 6 1.01= 6.06 g
Mass of O = 1 16.00= 16.00 g
Mass of 1 mol of C_{2}H_{5}OH
= 46.08 g
Now use the formula you learned above to find the percent
compositions of the constituent elements:
Mass percent of C: 100% = 52.14%
Mass percent of H: 100% = 13.15%
Mass percent of O: 100% = 34.77%
Not so bad, right?
