Overview
This lecture explains how to calculate percent composition by mass for elements in a compound, using molecular formulas, atomic masses, and worked examples.
Percent Composition Formula
- Percent composition is calculated as: (Mass of element / Total mass of compound) × 100.
- You can use either the masses in grams or molar masses (g/mol) for the calculation.
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%
- The percentages should add up to 100%.
Example: C₅H₉NO₂
- 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 = 12.17%
- Oxygen: 32 / 115.132 × 100 = 27.8%
Comparing Percent Carbon in Different Compounds
- CH₄: (12.01 / 16.042) × 100 = 74.87%
- C₂H₇N: (24.02 / 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 by mass of carbon among the options.
Finding Molar Mass Given Mass Percent and Atoms
- If a compound has one nitrogen atom and 13.32% nitrogen by mass:
(14.01 / Molar mass) × 100 = 13.32%
- Rearranged: Molar mass = 14.01 / 0.1332 = 105.18 g/mol.
Key Terms & Definitions
- Percent Composition — The percentage by mass of each element in a compound.
- Molar Mass — The mass of one mole of a substance, usually in g/mol.
- Atomic Mass — The mass of a single atom of an element, in atomic mass units (u) or g/mol.
Action Items / Next Steps
- Practice calculating percent composition for assigned compounds.
- Review periodic table for atomic masses.
- Complete any related homework problems.