Deriving Molecular Formulas

Introduction

• Empirical Formulas: Indicate numerical ratio of atoms of different elements in a compound.
• Molecular Formulas: More precise, showing exact number of atoms of each element.

Importance

• Empirical formulas provide multiple possibilities due to their ratio-based nature.
• Molecular formulas eliminate ambiguity by providing exact atom counts.

Steps to Derive Molecular Formulas

1. Determine Molecular Mass

   • Essential to find the total atomic mass of the compound.

2. Use Empirical Formula

   • Empirical formula is derived from mass or percent composition of elements.

3. Calculate Empirical Formula Mass

   • Add atomic masses of elements in the empirical formula.
   • Example: CH2O = 12 (C) + 2 (H) + 16 (O) = 30 amu.

4. Compute Molecular Formula

   • Divide molecular mass by empirical formula mass.
   • Determine how many empirical units are needed to form the molecular formula.
   • Example: 180 amu (molecular mass) / 30 amu (empirical formula mass) = 6.

5. Formulate Molecular Formula

   • Multiply each atom in the empirical formula by the number of empirical units.
   • Example: CH2O x 6 = C6H12O6.

Example Calculation

• Empirical Formula: CH2O
• Empirical Formula Mass: 30 amu
• Molecular Mass: 180 amu
• Molecular Formula: C6H12O6
  • Calculation: 180 / 30 = 6
  • Multiply empirical formula by 6.

Conclusion

• Successfully deriving a molecular formula provides precise atom counts and removes ambiguity inherent in empirical formulas.