Percent Composition Formula
The percent composition formula calculates the mass percentage of each element in a compound.
Essential for chemical analysis and empirical formulas.
The Formula
Percent composition tells you what fraction of a compound's total mass comes from each element. It is expressed as a percentage.
Variables
| Symbol | Meaning |
|---|---|
| % composition | Mass percentage of the element in the compound |
| mass of element | Total mass of that element in one mole of the compound (g) |
| total mass of compound | Molar mass of the compound (g/mol) |
Example 1
What is the percent composition of hydrogen and oxygen in water (H₂O)?
Molar mass of H₂O = 2(1.008) + 16.00 = 18.016 g/mol
% hydrogen = (2.016 / 18.016) × 100 = 11.19%
% oxygen = (16.00 / 18.016) × 100 = 88.81%
Water is 11.19% hydrogen and 88.81% oxygen by mass
Example 2
What is the percent composition of carbon in carbon dioxide (CO₂)?
Molar mass of CO₂ = 12.01 + 2(16.00) = 44.01 g/mol
% carbon = (12.01 / 44.01) × 100
Carbon makes up 27.29% of CO₂ by mass
When to Use It
Use percent composition to analyze the makeup of chemical compounds.
- Determining the mass percentage of each element in a compound
- Verifying the purity of a substance
- Finding empirical formulas from experimental data
- Nutritional analysis (percentage of elements in food compounds)
Key Notes
- Formula: % element = (mass of element in 1 mol / molar mass of compound) × 100: Use atomic masses from the periodic table. For example, in H₂O: H contributes 2×1.008 = 2.016 g/mol out of 18.015 g/mol total.
- Percentages must sum to 100%: Add up the percent composition of all elements in the compound. Any deviation from 100% (beyond rounding) indicates a calculation error.
- Used to find empirical formulas: Given percent composition data, divide each percentage by the element's atomic mass to get a mole ratio, then reduce to the simplest whole-number ratio.
- Isomers have the same percent composition: Ethanol (C₂H₅OH) and dimethyl ether (CH₃OCH₃) have the same molecular formula (C₂H₆O) and the same percent composition, but very different structures and properties.
- Accounts for isotope abundance: Atomic masses on the periodic table are weighted averages of all naturally occurring isotopes. For example, carbon's atomic mass of 12.011 reflects the 1.1% abundance of ¹³C.