3.5 Empirical Formulas

Empirical Formulas

  • Definition: The empirical formula is the simplest whole number ratio of elements in a chemical compound.

    • Example: For benzene (C₆H₆), the empirical formula is CH.

  • Illustration of Benzene:

    • Benzene can be represented in different ways:

      • Full structural formula showing all atoms.

      • Simplified 6-membered ring notation.

      • Molecular formula (C₆H₆) reflects the exact number of atoms.

  • Empirical vs. Molecular Formula:

    • Empirical formula shows the simplest ratio (C:H = 1:1 for benzene).

    • Molecular formula indicates the actual number of each type of atom.

Determining Empirical Formula from Percent Composition

  • Percent Composition: To find the empirical formula from the percent composition of elements.

    • Given example: 30.45% nitrogen and 69.56% oxygen.

    • These percentages sum to 100%.

  • Assuming 100 g Sample:

    • 30.45 g nitrogen and 69.56 g oxygen based on a 100 g sample assumption.

  • Conversion to Moles:

    • Calculate moles of nitrogen:

      • Moles of N = 30.45 g / 14.01 g/mol = 2.173 moles.

    • Calculate moles of oxygen:

      • Moles of O = 69.56 g / 16 g/mol = 4.3475 moles.

  • Mole Ratio Calculation:

    • Use the smaller mole value to normalize the mole ratio:

      • Nitrogen: 2.173 / 2.173 = 1

      • Oxygen: 4.3475 / 2.173 ≈ 2

    • Resulting empirical formula: NO₂ (1 nitrogen for every 2 oxygens).

Relationship Between Empirical and Molecular Formulas

  • Empirical Formula vs. Molecular Formula:

    • Empirical formula reflects the ratio while molecular formula provides actual counts.

    • Example: Acetylene (C₂H₂) also has the empirical formula CH.

  • Calculating Molecular Formula:

    • Given the empirical formula (NO₂), determine the molecular formula.

    • Calculate molar mass of NO₂:

      • Molar mass of NO₂ = 14.01 (N) + 2 * 16.00 (O) = 46.01 g/mol.

    • Compare with molecular weight (e.g., 92.02 g/mol).

      • Determine multiple: 92.02 g/mol / 46.01 g/mol = 2.

  • Final Molecular Formula Calculation:

    • Multiply empirical formula subscripts by 2:

      • Resulting molecular formula = N₂O₄.

  • Visual Representation:

    • Distinction between NO₂ and N₂O₄ to illustrate differences despite having the same empirical formula.

Concept Check

  • Engage in exercises to reinforce understanding of empirical vs molecular formulas, calculation methods, and examples provided in the segment.