Percent Composition
Percent composition shows each element's share of mass.
About this topic
Percent composition is the percentage by mass of each element in a compound: (mass of element in one mole ÷ molar mass of compound) × 100. For water, hydrogen is 2.02/18.02 = 11.2 % and oxygen 88.8 %. It is used to identify unknown compounds and verify purity.
Because the composition of a pure compound is fixed, the percentages are the same for any sample size, a statement known as the law of definite proportions. The percentages should add to about 100 percent, which gives a handy check. To calculate, find the molar mass, then divide the total mass of each element (atomic mass times number of atoms) by it. Experimental data can be converted the other way: if a 10.0 g sample contains 4.0 g of carbon, the carbon percentage is 40 percent.
Example: in iron(III) oxide, Fe2O3, the molar mass is 159.7 g/mol, with 111.7 from iron, giving about 69.9 percent iron. An ore company can use this to see how much iron it could get from a tonne of ore, while fertiliser buyers compare nitrogen percentages: urea is about 46 percent nitrogen and ammonium nitrate about 35.
A common error is using atomic masses without multiplying by the subscripts, or dividing by the wrong total. Always check that the percentages sum to 100.
Key ideas
- Mass of element ÷ molar mass × 100
- Parts add to 100%
- Used to find formulas
Quick check
Use cases: where Percent Composition helps
- Homework: review the key idea behind Percent Composition before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Percent Composition without notes.
- Visual learning: play with the interactive lab to see how Percent Composition changes when you adjust its inputs.
- Teaching a friend: explain Percent Composition in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Percent Composition?
Percent composition shows each element's share of mass.
What will I learn in Percent Composition?
Mass of element ÷ molar mass × 100. Parts add to 100%. Used to find formulas.
Is Percent Composition part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Percent Composition lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Percent Composition?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.