Chemistry 11 · Chemistry · Lesson 4.4
Percentage composition
- Level
- Grade 11
- Reading time
- 11 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Finding the mass percent of each element in a compound.
What is percent composition?
Short answer
Percent composition gives the percentage by mass of each element in a compound. You find it by dividing the total mass of each element in one mole of the compound by the compound’s molar mass and multiplying by 100. For example, water is about 11% hydrogen and 89% oxygen by mass. It is the first step toward finding an empirical formula.
Concept preview
What you will be able to do
- 01Explain what percentage composition means
- 02Calculate the mass percent of each element in a compound
- 03Find the percent of water in a hydrate
The percentage composition of a compound is the percentage, by mass, of each element in its formula. Because a compound has a fixed formula, its percentage composition is a constant — every sample of pure water is hydrogen and oxygen by mass, no matter where it came from. Percentage composition is a useful fingerprint and the starting point for finding an unknown's formula.
School Plan
Percentage composition
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Related topics
Frequently asked questions
- How do you calculate percent composition?
- Divide the mass of each element in one mole of the compound by the molar mass, then multiply by 100.
- Why is percent composition useful?
- It lets you compare compounds by mass and is the starting point for determining an empirical formula from experimental data.