Introduction to Molecular and Empirical Formulas

Overview

• Discuss the definition and differences between molecular and empirical formulas.
• Learn how to write an empirical formula from a molecular formula.

Molecular Formula

• Represents the actual number of atoms of each element in a compound.
• Example: Ethene (C4H8)
  • Carbon: 4 atoms (C4)
  • Hydrogen: 8 atoms (H8)

Empirical Formula

• Represents the simplest or most reduced ratio of atoms in a compound.
• Example: Ethene
  • Ratio of Carbon to Hydrogen: 4:8
  • Simplification: 4/8 = 1/2 (divide by 4)
  • Empirical Formula: CH2

Example: Cyanotriazide

• Molecular Formula: C3N12
  • Carbon: 3 atoms (C3)
  • Nitrogen: 12 atoms (N12)
• Empirical Formula
  • Ratio of Carbon to Nitrogen: 3:12
  • Simplification: 3/12 = 1/4 (divide by 3)
  • Empirical Formula: CN4

More Complex Examples

• For molecular formulas with multiple elements (e.g., C5H10O5), find the largest number to simplify:
  • Example: C5H10O5
  • Largest divisor: 5
  • Empirical Formula: CH2O

When Simplification is Not Possible

• Some molecular formulas cannot be simplified.
  • Example: P3N5 and C5H12
  • Empirical Formula is the same as the molecular formula.

Multiple Compounds with Same Empirical Formula

• Different compounds can have identical empirical formulas.
  • Example: C4H8 has CH2 as its empirical formula, as do:
    • C2H4
    • C3H6
    • C5H10
    • C6H12
• Empirical formula indicates a simple ratio among those compounds.

Conclusion

• Molecular Formula: Total number of atoms of each element in a compound.
• Empirical Formula: Simplest ratio of those atoms.
• Some molecular formulas cannot simplify further, making the empirical formula the same as the molecular one.
• Many compounds can share the same empirical formula.