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 C2H6) and in terms of the percentages (by mass) of its
elements.
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, C2H5OH.
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
Mass of H = 6
1.01
= 24.02 g
= 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:
Not so bad, right?
Mass percent of C:
100% = 52.14%
Mass percent of H:
100% = 13.15%
Mass percent of O:
100% = 34.77%
Another way to look at it and more sample problems:
Total mass of the Element (Part) x
Total mass of compound (Whole)100%
= Percent Composition
Example 1
Calculate the percent by weight of sodium (Na) and chlorine (Cl) in sodium
chloride (NaCl)
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Calculate the molecular mass (MM):
MM = 22.99 + 35.45 = 58.44
Calculate the total mass of Na present:
1 Na is present in the formula, mass = 22.99
Calculate the percent by weight of Na in NaCl:
%Na = (mass Na ÷ MM) x 100 = (22.99 ÷ 58.44) x 100 = 39.34%
Calculate the total mass of Cl present:
1 Cl is present in the formula, mass = 35.45
Calculate the percent by weight of Cl in NaCl:
%Cl = (mass Cl ÷ MM) x 100 = (35.45 ÷ 58.44) x 100 = 60.66%
The answers above are probably correct if %Na + %Cl = 100, that is,
39.34 + 60.66 = 100.
Example 2
Calculate the percent by weight of each element present in sodium sulfate
(Na2SO4).
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Calculate the molecular mass (MM):
MM = (2 x 22.99) + 32.06 + (4 x 16.00) = 142.04
Calculate the total mass of Na present:
2 Na are present in the formula, mass = 2 x 22.99 = 45.98
Calculate the percent by weight of Na in Na2SO4:
%Na = (mass Na ÷ MM) x 100 = (45.98 ÷ 142.04) x 100 = 32.37%
Calculate the total mass of S present in Na2SO4:
1 S is present in the formula, mass = 32.06
Calculate the percent by weight of S present:
%S = (mass S ÷ MM) x 100 = (32.06 ÷ 142.04) x 100 = 22.57%
Calculate the total mass of O present in Na2SO4:
4 O are present in the formula, mass = 4 x 16.00 = 64.00
Calculate the percent by weight of O in Na2SO4:
%O = (mass O ÷ MM) x 100 = (64.00 ÷ 142.04) x 100 = 45.06%
The answers above are probably correct if %Na + %S + %O = 100, that is,
32.37 + 22.57 + 45.06 = 100