Overview
This lecture explains how to calculate percent composition by mass of elements in chemical compounds, using examples and formulas.
Percent Composition Formula
- Percent composition (mass percent) = (mass of element / total mass of compound) × 100.
- You can use either actual mass or molar mass from the periodic table; results are the same.
Example: Sucrose (C₁₂H₂₂O₁₁)
- Find total molar mass: (12 × 12.01) + (22 × 1.008) + (11 × 16) = 342.296 g/mol.
- Carbon: (12 × 12.01) / 342.296 × 100 = 42.1%
- Hydrogen: (22 × 1.008) / 342.296 × 100 = 6.5%
- Oxygen: (11 × 16) / 342.296 × 100 = 51.4%
- Percentages add up to 100%, confirming accuracy.
Example: C₅H₉NO₂
- Total molar mass: (5 × 12.01) + (9 × 1.008) + (1 × 14.01) + (2 × 16) = 115.132 g/mol.
- Carbon: 60.05 / 115.132 × 100 = 52.16%
- Hydrogen: 9.072 / 115.132 × 100 = 7.88%
- Nitrogen: 14.01 / 115.132 × 100 = 12.1%
- Oxygen: 32 / 115.132 × 100 = 27.8%
Comparative Example: Greatest Percent Carbon by Mass
- Calculate percent carbon for each compound:
- CH₄: 12.01 / 16.042 × 100 = 74.87%
- C₂H₇N: 24 / 45.086 × 100 = 53.3%
- C₅H₁₂O: 60.05 / 88.146 × 100 = 68.1%
- C₆H₁₂O₆: 72.06 / 180.156 × 100 = 40%
- CO₂: 12.01 / 44.01 × 100 = 27.3%
- CH₄ has the greatest percent carbon by mass.
Determining Unknown Molar Mass from Percent Composition
- If percent of nitrogen = 13.32% and molecule contains 1 N atom:
- 13.32 = (14.01 / x) × 100 → x = 14.01 / 0.1332 = 105.18 g/mol.
- Use percent = (mass of element / molar mass) × 100 and algebra to solve for unknown.
Key Terms & Definitions
- Percent Composition — Percentage by mass of each element in a compound.
- Molar Mass — Mass in grams of one mole of a substance, calculated from atomic masses.
Action Items / Next Steps
- Practice calculating percent composition for several compounds.
- Review molar masses and atomic masses using the periodic table.