Overview
This lecture explains the step-by-step process to find a compound's molecular formula using empirical formula calculations, demonstrated with a nicotine example.
Empirical Formula vs. Molecular Formula
- An empirical formula shows the simplest whole-number ratio of elements in a compound.
- A molecular formula gives the actual number of atoms of each element in one molecule of the compound.
Steps to Find the Molecular Formula
1. Convert Percentages to Grams
- Treat the given percentages as grams (e.g., 74.0% becomes 74.0g).
2. Convert Grams to Moles
- Use the molar mass of each element:
- Carbon (C): 12.01 g/mol, Hydrogen (H): 1.01 g/mol, Nitrogen (N): 14.01 g/mol.
- Calculate moles for each element by dividing grams by molar mass.
3. Find Simplest Mole Ratio
- Divide all mole values by the smallest number of moles found.
4. Write the Empirical Formula
- Use the resulting whole numbers as subscripts for C, H, and N in the formula.
5. Calculate Empirical Formula Molar Mass
- Multiply each element's subscript by its molar mass and add the totals.
6. Determine the Molecular Formula
- Divide the given molar mass of the compound by the empirical formula’s molar mass.
- Multiply the empirical formula’s subscripts by the resulting whole number to get the molecular formula.
7. Example: Nicotine
- Empirical formula found: C₅H₇N.
- Empirical molar mass: 81.13 g/mol.
- Molecular formula calculation: 162.1 g/mol ÷ 81.13 g/mol = 2.
- Molecular formula: C₁₀H₁₄N₂.
Key Terms & Definitions
- Empirical Formula — Simplest whole-number ratio of elements in a compound.
- Molecular Formula — Actual number of each atom in a molecule.
- Molar Mass — Mass of one mole of a substance, in grams per mole.
Action Items / Next Steps
- Practice calculating empirical and molecular formulas with more examples.
- Review steps if needed and rewatch instructional content.
- Consider scheduling live tutoring for additional help.