Percent Composition
A compound consists of a specific ratio of two or more elements. Because of this, it is possible to calculate the percent composition by mass of a compound. The percent composition is constant for the compound. Percent composition is calculated using the total mass of a compound as the “whole” or “total.” Then the individual masses of each element type represent the “part.”
Calculating Percent Composition
Tricalcium phosphate (Ca3(PO4)2) is found naturally throughout the world in rock, in the skeletons of animals, and in the milk you drink. Examine the process below to determine the percent of calcium in tricalcium phosphate, Ca3(PO4)2.
To calculate the percentage of calcium in Ca3(PO4)2:
First, calculate the molar mass, or gram formula mass, of the compound:
- 3 atoms of Ca: 3 (40.08 g/mol) = 120.24 g/mol
- 2 atoms of P: 2 (30.97376 g/mol) = 61.94752 g/mol
- 8 atoms of O: 8 (15.9994 g/mol) = 127.9952 g/mol
Then, add the products together to get the molar mass of the "whole."
120.24 g/mol + 61.94752 g/mol + 127.9952 g/mol = 310.18 g/mol.
The "part" of calcium in Ca3(PO4)2 has a molar mass of 120.24 g/mol, so you need to divide the "part" by the "whole" and then multiply by 100 to get the percent of Ca in the compound.
The percent of calcium in Ca3(PO4)2 is 38.765%.
Percent Composition Review
Now that you have learned how to calculate the percent composition of a compound, review your knowledge in this non-graded activity. Read the directions associated with each question and provide or select the correct answer. Then, click SUBMIT to check your response. Click the interactivity thumbnail and then click NEXT to get started.