Percent Composition Examples in Chemistry
Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Percent Composition.
This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.
Concept Recap
The percentage by mass that each element contributes to the total molar mass of a compound, calculated by dividing the total mass of each element in one mole of the compound by the compound's molar mass and multiplying by 100.
What fraction of the compound's total weight is made up by each element inside it.
Read the full concept explanation →How to Use These Examples
- Read the first worked example with the solution open so the structure is clear.
- Try the practice problems before revealing each solution.
- Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea: Percent composition is a fixed ratio for any pure compound, regardless of the sample size.
Common stuck point: Use molar masses (g/mol from the periodic table), not atomic numbers, in the calculation.
Sense of Study hint: When calculating percent composition, work element by element. First find the molar mass of the entire compound by summing all atomic masses. Then for each element, multiply its atomic mass by the number of atoms of that element in the formula. Finally, divide by the total molar mass and multiply by 100.
Worked Examples
Example 1
easySolution
- 1 Molar mass of \text{H}_2\text{O} = 2(1.008) + 16.00 = 18.02\,\text{g/mol}.
- 2 %H = \frac{2(1.008)}{18.02} \times 100\% = \frac{2.016}{18.02} \times 100\% = 11.19\%.
- 3 %O = \frac{16.00}{18.02} \times 100\% = 88.81\%.
Answer
Example 2
mediumPractice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
easyExample 2
easyRelated Concepts
Background Knowledge
These ideas may be useful before you work through the harder examples.