1. Chapter 6 Chemical Quantities
6.6
Percent Composition

2. What is percentage composition?
The mass of each element in a
compound compared to the entire
mass of the compound and
multiplied by 100 percent is called
the percentage composition of the
compound.

3. To calculate percentage composition:
Mass of element X = %
Total mass of compound
This calculation can be performed for a chemical with a known formula Or
With laboratory data

4. Percent Composition – From a Chemical Formula
Example: Calculate the percent composition of CO2.
Step 1: Calculate the molar mass of each element involved.
C: 12.01g/mol 2 O: 2 x(16.00g/mol) = 32.0g/mol
Step 2: Calculate the total molar mass of compound.
M CO2 = 12.01g/mol g/mol = 44.01g/mol
Step 3: Determine precentages
12.01 g/mol C x = % C
44.01 g/mol CO2
32.00 g/mol O x = % O

5. Learning Check
What is the percent composition of lactic acid, C3H6O3, a compound that appears in the blood after vigorous activity?

6. Solution STEP 1 3C(12.01) + 6H(1.01) + 3O(16.00) STEP 2
36.03 g C g H g O = g/mol
Step 3: Determine precentages
%C = g C x 100 = 40.00% C
90.08 g
%H = g H x 100 = 6.714% H
%O = g O x 100 = % O

7. Learning Check
The chemical isoamyl acetate C7H14O2 gives the odor of pears. What is the percent carbon in isoamyl acetate?
1) %C
2) %C
3) %C

8. Solution 3) 64.58 %C Molar mass C7H14O2 = 7C(12.01) + 14H(1.008)
+ 2O(16.00) = g/mol
Total C = 7C(12.01) = g
% C = total g C x 100
total g
% C = g C x 100 = % C g

9. From Lab Data Find the percentage composition of a
compound that contains 2.63g of
carbon, 0.370g of hydrogen, and
0.580g of oxygen in a 3.58g sample
of the compound.

10. Solution % C = 2.63g × 100 = 73.46% 3.58g % H = 0.370g × 100 = 10.33%
% O = 0.580g × 100 = %
Therefore the unknown compound is 73.46% carbon, % hydrogen and 16.20% oxygen.