What is an example of a percent composition practice problem?

1 Answer
Jun 3, 2014

To calculate the percent composition, you need to know the mass of each individual component in your compound. This includes the mass of each element in the compound, the total mass of the compound, and the mass of each element added together.

The formula for percent composition is:

% by mass = (mass of element in the compound/ total mass of the compound) X 100

Here is an example:

What is the percent composition of table sugar (C12H22O11)?

Step 1:
First multiply the number of each type of element by its atomic mass

Carbon = 12 X 12.01 = 144.13 grams/mole
Hydrogen = 22 X 1.0079 = 22.17 grams/mole
Oxygen = 11 X 15.999 = 175.99 grams/mole
The total mass is 342.29 grams/mole

Step 2:
Divide each element's grams/mole by the total grams/mole and multiply the answers by 100.

%C = (144.13/342.29) X 100 = 42.1%
%H = (22.17/342.29) X 100 = 6.5%
%O = (175.99/342.29) X 100 = 51.4%

You would follow the same procedure to find the percent composition of any substance but you need to know how much of each individual component you have and the total mass.