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Understanding Combustion Analysis Techniques

Sep 11, 2024

Combustion Analysis for Empirical and Molecular Formulas

Introduction to Combustion Analysis

  • Combustion analysis is used to determine empirical and molecular formulas.
  • It is similar to elemental analysis but involves combustion with oxygen (O2).
  • Products are carbon dioxide (CO2) and water (H2O).
  • Assumes sample contains only carbon, hydrogen, and oxygen.

Strategy for Combustion Analysis

  • Step 1: Determine mass of carbon from CO2 produced.
  • Step 2: Determine mass of hydrogen from H2O produced.
  • Step 3: Calculate mass of oxygen using the mass of the original sample and conservation of mass principles.
  • Steps 4-6: Same as elemental analysis.
    • Calculate moles of each element.
    • Write a preliminary formula.
    • Determine empirical and (if possible) molecular formulas.

Example: Vitamin C Analysis

  • Molar Mass of Vitamin C: 176.12 g/mol
  • Data Provided:
    • 1.50 grams of CO2 produced.
    • 0.41 grams of H2O produced.
    • Total sample mass: 1 gram.

Step-by-Step Calculation

Step 1: Mass of Carbon

  1. Use mass of CO2 (1.50 g) to find grams of carbon.
  2. Convert grams of CO2 to moles using CO2 molar mass (44.01 g/mol).
  3. Use stoichiometry: 1 mole of CO2 = 1 mole of C.
  4. Calculate grams of carbon.

Step 2: Mass of Hydrogen

  1. Use mass of H2O (0.41 g) to find grams of hydrogen.
  2. Convert grams of H2O to moles using H2O molar mass.
  3. Critical step: 2 moles of H per 1 mole of H2O.
  4. Calculate moles and grams of hydrogen.

Step 3: Mass of Oxygen

  • Cannot use CO2 or H2O to calculate oxygen (originates from O2 used in combustion).
  • Mass of sample (1 g) = Mass of C + Mass of H + Mass of O.
  • Calculate mass of oxygen from remaining mass.

Step 4: Moles of Each Element

  • Carbon and hydrogen moles saved from previous steps.
  • Calculate moles of oxygen from its mass (0.545 g divided by 15.999 g/mol).

Step 5: Empirical Formula

  1. Preliminary formula using mole values:
    • C: 0.0341 moles
    • H: 0.046 moles
    • O: 0.0341 moles
  2. Divide all subscripts by smallest number (0.0341).
  3. Resulting formula: C_1H_1.34O_1.
  4. Adjust H to whole number by trial (multiply by 3): C_3H_4O_3.

Step 6: Determine Molecular Formula

  • Calculate molar mass of empirical formula (88.032 g/mol).
  • Compare with actual molar mass (176.12 g/mol).
  • Factor of 2 needed, hence molecular formula: C_6H_8O_6.

Conclusion

  • Combustion analysis provides a systematic method to determine empirical and molecular formulas using the combustion reaction byproduct data and conservation of mass principles.
  • Requires careful calculation and checking for whole number ratios in empirical formulas.

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