Chemistry / Unit 5: Chemical Names and Formulas
5.5 Percent Composition
Notes
Percent composition is the percent by mass of each element in a compound.
- $\displaystyle \% \text{ element} = \frac{\text{mass of element in 1 mol}}{\text{molar mass of compound}} \times 100\%$.
- Find the molar mass by adding atomic masses (H 1.01, C 12.01, O 16.00, etc.).
- Example: $\text{H}_2\text{O}$ has molar mass $2(1.01) + 16.00 = 18.02$ g/mol. % H $\displaystyle = \frac{2.02}{18.02} \times 100\% = 11.2\%$, and % O $= 88.8\%$.
- The percents of all elements add to 100%.
Practice
What is the percent by mass of carbon in $\text{CO}_2$? (C = 12.01, O = 16.00)
Molar mass $= 12.01 + 32.00 = 44.01$ g/mol. $\displaystyle \frac{12.01}{44.01} \times 100\% = 27.3\%$. 33.3% counts atoms instead of mass.
What is the percent by mass of iron in $\text{Fe}_2\text{O}_3$? (Fe = 55.85, O = 16.00)
Molar mass $= 2(55.85) + 3(16.00) = 159.70$ g/mol. $\displaystyle \frac{111.70}{159.70} \times 100\% = 69.94\%$.
What is the percent by mass of sodium in NaCl? (Na = 22.99, Cl = 35.45)
$\displaystyle \frac{22.99}{58.44} \times 100\% = 39.34\%$.
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