Overview
This lecture explains how to determine the empirical formula of a compound using percentage composition, including a step-by-step worked example.
What is an Empirical Formula?
- An empirical formula shows the simplest whole-number ratio of elements in a compound.
- It does not provide the exact number of atoms found in the compound’s molecular formula.
Steps to Calculate an Empirical Formula
- Step 1: Convert percentage composition directly to grams (assume 100 g sample).
- Step 2: Convert grams of each element to moles using their molar masses.
- Step 3: Divide all mole values by the smallest number of moles calculated.
- Step 4: If any subscripts are not whole numbers, multiply all subscripts by the smallest number that turns them into whole numbers.
Example Problem: Chromium and Oxygen Compound
- Given: 68.4% Chromium, 31.6% Oxygen.
- Convert to grams: 68.4 g Cr, 31.6 g O.
- Convert to moles: 68.4 g Cr / (molar mass Cr) = 1.315 mol Cr; 31.6 g O / (molar mass O) = 1.975 mol O.
- Divide by smallest: 1.315 mol Cr / 1.315 = 1; 1.975 mol O / 1.315 = 1.5.
- Decimal ends in .5, so multiply both by 2: Cr₂O₃ is the empirical formula.
Multiplying Subscripts Rule
- Decimal ends in 0.5: multiply all subscripts by 2.
- Decimal ends in 0.33 or 0.66: multiply by 3.
- Decimal ends in 0.25 or 0.75: multiply by 4.
Key Terms & Definitions
- Empirical Formula — simplest whole-number ratio of elements in a compound.
- Molar Mass — mass of one mole of an element, used to convert grams to moles.
Action Items / Next Steps
- Practice similar empirical formula problems.
- Review molar mass calculations.
- Check the description for live tutoring session details.