Lecture Notes: Determining Chemical Formulas from Experimental Data

Introduction

• Topic: Chemical formulas from experimental data
• Focus: Elemental analysis
• Use Case: Confirms the identity of synthesized compounds or unidentified samples like archaeological finds

Elemental Analysis

• Purpose: Conversion of mass percentages to chemical formulas
• Requirement: Pure sample is necessary
• Steps in the Process:
  1. Convert mass percentages to grams
  2. Convert grams to moles
  3. Write a preliminary formula
  4. Convert to an empirical formula (smallest whole number ratios)
  5. If possible, determine molecular formula

Empirical vs. Molecular Formula

• Empirical Formula: Simplest whole number ratio (e.g., C2H4 becomes CH2)
• Molecular Formula: Actual number of atoms in a molecule (more detailed than empirical)

Example Problem

• Given:
  • Molar mass: 90.08 g/mol
  • Mass composition: 40% C, 6.71% H, 53.3% O
• Objective: Determine molecular formula

Step-by-Step Solution

1. Convert percentages to grams: Assume a 100g sample for simplicity

   • 40% C = 40g of C
   • 6.71% H = 6.71g of H
   • 53.3% O = 53.3g of O

2. Convert grams to moles:

   • Use molar mass of each element from periodic table

3. Preliminary formula:

   • Calculate mole ratios (e.g., 3.33 moles of C, etc.)

4. Convert to integers:

   • Divide by smallest mole value to get empirical formula
   • Example: If smallest value is 3.33, divide all by 3.33
   • Result: Empirical formula CH2O

5. Determine molecular formula:

   • Calculate empirical molar mass
   • Compare to given molar mass
   • Example: C3H6O3 (multiplication factor of 3)

Conclusion

• Significance: Empirical formulas provide a foundational understanding, while molecular formulas give the complete picture
• Practical Application: Requires knowledge of molar mass for full molecular formula