What is Percent Composition?
Percent composition is the mass percentage of each element in a compound. It's calculated as (mass of element in formula / molar mass of compound) x 100%. For example, water (H?O, MW = 18.015 g/mol) contains 2(1.008) = 2.016 g hydrogen and 15.999 g oxygen. Hydrogen is (2.016/18.015) x 100% = 11.19%, oxygen is 88.81%. Percent composition is independent of sample size-it's an intrinsic property of the compound.
Calculating Percent Composition
To calculate percent composition: determine the compound's molar mass, find the total mass of each element in the formula (atomic mass x subscript), divide each element's mass by total molar mass, and multiply by 100%. For H?SO? (MW = 98.08 g/mol): H = 2(1.008)/98.08 x 100% = 2.06%, S = 32.06/98.08 x 100% = 32.69%, O = 4(16.00)/98.08 x 100% = 65.25%. Sum should equal 100% (within rounding error).
Applications and Importance
Percent composition helps verify compound identity and purity. Chemists use it to determine empirical formulas from experimental data. Environmental scientists measure elemental content in soil, water, and air samples. Metallurgists analyze alloy compositions. Nutritionists determine mineral content in foods. Forensic scientists identify unknown substances. Quality control laboratories verify that products meet specifications. Understanding percent composition is essential for elemental analysis, stoichiometry problems, and converting between empirical and molecular formulas.
Quick Tips
- Always verify units are consistent
- Use scientific notation for very large/small numbers
- Results are approximations — real conditions may vary
Frequently Asked Questions
Percent composition gives mass percentages of elements. Empirical formula gives the simplest whole-number ratio of atoms. You can calculate percent composition from empirical formula and vice versa.
Yes, if they have the same empirical formula. For example, CH?O (formaldehyde), C?H?O? (acetic acid), and C?H??O? (glucose) all have empirical formula CH?O and identical percent composition: 40.0% C, 6.7% H, 53.3% O.
Sum all percentages-they should equal 100% within rounding error. If significantly different, check calculations. Small discrepancies (<0.5%) are usually rounding errors. Large discrepancies indicate mistakes.
No, only empirical formula. Percent composition gives element ratios, not actual numbers of atoms. You need additional information (like molar mass) to determine molecular formula from empirical formula.
Compare experimental percent composition to theoretical. Deviations indicate impurities or incorrect formula. For example, if measured iron content in Fe?O? is lower than theoretical 69.94%, the sample may contain impurities or isn't pure Fe?O?.