Overview
This lecture explains the difference between molecular and empirical formulas, how to identify and calculate each, and methods for converting between them.
Molecular vs. Empirical Formula
- A molecular formula shows the actual number and type of each atom in a molecule (e.g., H₂O means two hydrogens and one oxygen).
- A molecular formula gives specific atom counts for a compound (e.g., CO₂ contains one carbon and two oxygen atoms).
- An empirical formula is the simplest whole-number ratio of atoms in a compound.
- For glucose (C₆H₁₂O₆), the empirical formula is CH₂O (as 6:12:6 simplifies to 1:2:1).
- The empirical formula reduces atom counts to their lowest whole-number ratio (e.g., P₄O₁₀ becomes P₂O₅).
Determining Empirical Formulas
- Simplify the subscripts in the molecular formula by dividing all by the largest common factor.
- If given atom ratios (e.g., 4 phosphorus to 10 oxygen), divide both numbers by their greatest common divisor to get the empirical formula.
- The empirical formula is always expressed in the lowest possible integer ratio.
Calculating Formulas from Composition
- Empirical formulas can be calculated from experimental mass or percentage composition data of elements.
- Once the empirical formula is found, it can be converted to the molecular formula if the compound's molar mass is known.
Key Terms & Definitions
- Molecular Formula — shows the exact number and type of atoms in a molecule.
- Empirical Formula — shows the simplest whole-number atom ratio in a compound.
Action Items / Next Steps
- Practice calculating empirical formulas from given masses or percent compositions.
- Learn to convert empirical formula to molecular formula using molar mass.