Lecture Notes: Mass Percents and Empirical Molecular Formulas

Introduction

• Topic: Mass percents and empirical molecular formulas
• Part of a high school chemistry series on stoichiometry
• Lecturer: Chad from Chad's Prep
• Lessons will be released weekly throughout the 2020-21 school year

Mass Percents

• Concept: Percentage of each element within a compound based on mass
• Formula: ( \text{Percent} = \frac{\text{grams of element}}{\text{total grams of sample}} \times 100 )
• Example: Calcium carbonate (CaCO3)
  • Convenient sample size: 1 mole (100 grams total)
  • Calculating Percentages:
    • Carbon: 12 grams of carbon in 1 mole, thus 12% carbon
    • Oxygen: 48 grams of oxygen in 1 mole, thus 48% oxygen

Intensive Property

• Mass percents are intensive properties
• Independent of sample size

Empirical vs. Molecular Formulas

• Molecular Formula: Actual number of atoms of each element in a molecule
• Empirical Formula: Simplest whole number ratio of atoms in a compound

Examples

• Benzene (C6H6): Empirical formula CH
• P4O10: Empirical formula P2O5
• Ethanol (C2H6O): Empirical formula same as molecular formula

Converting Mass Percents to Empirical Formula

• Example: Compound with 80% carbon and 20% hydrogen by mass
  • Start with 100 gram sample size for easier math
  • Convert mass to moles:
    • Carbon: 80 grams -> 6.67 moles (12 g/mol)
    • Hydrogen: 20 grams -> 20 moles (1 g/mol)
  • Empirical Formula Calculation:
    • Ratio: C6.67H20
    • Simplify by dividing by smallest number (6.67)
    • Empirical formula: CH3

Handling Non-Whole Number Ratios

• If ratio is not a whole number (e.g., C1H2.5)
  • Multiply to get whole numbers (e.g., multiply by 2 to get C2H5)

Determining Molecular Formula

• Empirical formula alone can't determine molecular formula
• Requires additional data: Molecular weight or molar mass
• Example: Given molecular weight of 30 AMUs
  • Empirical weight of CH3: 15
  • Molecular formula: Double empirical formula to C2H6

Conclusion

• Understanding mass percents and empirical formulas is essential for stoichiometry
• Practice problems and further resources available on Chad's Prep website

Additional Resources

• Chad's Prep website for practice problems and study guides: chadsprep.com

Feel free to leave questions in the comment section of the lecture video.