Percent Composition
Calculator
Results
- Percent composition (%)
- 27.292144
- Remaining fraction (%)
- 72.707855
Chemistry results
| Percent composition (%) | 27.292144 |
| Remaining fraction (%) | 72.707855 |
formula-map diagram
- Percent composition (%)
- 27.292144
- Remaining fraction (%)
- 72.707855
Formula breakdown
Formula
% composition = (element mass ÷ compound mass) × 100= 27.292144788566
Note
Simplified model: these results assume ideal behaviour (ideal gases, dilute solutions, complete reactions) and standard textbook constants. Real laboratory values vary with temperature, pressure, purity and activity coefficients. Do not rely on this for safety-critical or analytical work.
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See all →Frequently asked questions
What does percent composition tell you about a compound?+
Percent composition shows the mass percentage that each element contributes to a compound's total molar mass, revealing what fraction of a sample's weight comes from each element. It's calculated as (mass of element in one mole of compound / molar mass of compound) x 100%.
How do I find the percent composition of a compound like water?+
For H2O (molar mass about 18.02 g/mol), hydrogen contributes 2 x 1.008 = 2.016 g, so hydrogen is about 11.2% by mass, and oxygen (16.00 g) makes up the remaining 88.8%. The percentages of all elements in a compound always add up to 100%.
How is percent composition used to find an empirical formula?+
Percent composition (often from experimental combustion analysis) is converted to grams per 100 g of sample, then to moles of each element, and the mole ratios are simplified to whole numbers to get the empirical formula. This is the standard reverse path from lab data to a chemical formula.
Why might a measured percent composition differ slightly from the calculated one?+
Impurities in the sample, incomplete reactions during analysis, or measurement error in the lab instruments can all cause small deviations between experimental and theoretical percent composition. Large deviations usually point to sample impurity or a different compound than expected.
Does percent composition depend on the sample size?+
No — percent composition is a ratio based on molar mass, so it's identical whether you have one gram or one kilogram of the compound. It describes the compound's fixed elemental makeup, not the size of a particular sample.