Percent Composition

Percent composition refers to the percentage by mass, not percentage by the number of atoms in the compound. For example - H 2 O is not 66.6% Hydrogen and 33.3% Oxygen. Percentage composition tells us what percentage the compound is based on the weight of the elements that make it up. It does not tell us the ratio of the atoms and does not tell us the subscripts.

Example 1 Homer has a dozen assorted doughnuts. 3 of these doughnuts are chocolate. What percentage of Homer's doughnuts are chocolate? (note, we are talking about total number, not mass, in this case.)

Example 2 Jim goes to a school that has 258 total students. If 164 of the students are boys, what percentage of the student body is made up of girls?

In both cases, you found the percent composition by dividing one part by the total and multiplying by 100. part % composition = x 100 whole

Finding the percentage composition is similar. You divide the mass of one element by the mass of the entire compound. Formula for Percent Composition: mass of one element % Composition = x 100 total mass of the compound

Example 3 What percentage of the mass of carbon dioxide (CO 2 ) is made up by the carbon? Mass of Carbon = 1 x g = Mass of Oxygen = 2 x g = g % comp. of Carbon = x g Answer = 27.3% Carbon

Example 4 What is the percentage composition of glucose (C 6 H 12 O 6 ) ? Start by determining the molar mass! C = 6 x 12.0 g = 72.0 g H = 12 x 1.01 g = 12.1 g O = 6 x 16.0 g = 96.0 g g

72.0 g % for Carbon = x 100 = 40.0% C 180 g 12.1 g % for Hydrogen = x 100 = 6.7 % H 180 g 96.0 g % for Oxygen = x 100 = 53.3 % O 180 g

Check your Answer! One way to check your answer is to make sure that all of the percentages add up to approximately 100%. (i.e. 40.0% + 6.7% % = 100%) Your total may be off by a few tenths of a percent, due to rounding.