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Combustion Analysis

Related Examples and Practice Problems

Additional Worked Out Examples/ Practice

  • Identifying classification types: Differentiation between elements, compounds or mixtures and homogeneous and heterogenous mixtures

  • Separation techniques: Selected and explaining limitation of appropriate separation

  • Relating Properties to Composition: Predicting classification based on descriptive properties

    and more …

Topic Summary & Highlights
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Core Concept

Combustion analysis is an analytical technique used to determine the amount of carbon, hydrogen, and sometimes oxygen in a compound. It is commonly used to analyze organic compounds and involves burning the compound in excess oxygen, which produces carbon dioxide (CO₂) and water (H₂O) as products.

Steps in Combustion Analysis

  1. Burn the Sample in Excess Oxygen:

    • The compound combusts completely in an oxygen-rich environment, producing CO₂ and H₂O.

    • The mass of CO₂ and H₂O produced is measured.

  2. Determine the Mass of Carbon and Hydrogen:

    • All carbon in the original compound ends up as carbon in CO₂, and all hydrogen ends up as hydrogen in H₂O.

    • Calculate the mass of carbon from CO₂ and the mass of hydrogen from H₂O.

  3. Calculate the Moles of Carbon and Hydrogen:

    • Use the masses of carbon and hydrogen to find their moles in the sample.

  4. Determine the Amount of Oxygen (If Present):

    • If the compound also contains oxygen, subtract the mass of carbon and hydrogen from the total sample mass to find the mass of oxygen.

    • Calculate the moles of oxygen from this mass.

  5. Find the Empirical Formula:

    • Divide the moles of each element by the smallest number of moles.

    • This gives the simplest whole-number ratio, which is the empirical formula of the compound.

Key Tips and Reminders

  • All Carbon Ends Up in CO₂: Convert the mass of CO₂ to the mass of carbon to find the amount of carbon in the original compound.

  • All Hydrogen Ends Up in H₂O: Convert the mass of H₂O to the mass of hydrogen to find the amount of hydrogen.

  • Use the Sample Mass to Find Oxygen (if applicable): For compounds with oxygen, subtract the mass of carbon and hydrogen from the total mass to find the oxygen content.

  • Always Simplify Ratios: Divide by the smallest number of moles to get the simplest whole-number ratio.

Combustion Analysis Table
Step Example Application (0.500 g sample produces 1.467 g CO₂ and 0.600 g H₂O)
1. Burn the Sample in Excess Oxygen Combust the 0.500 g sample in oxygen, which yields 1.467 g of CO₂ and 0.600 g of H₂O.
2. Determine the Mass of Carbon from CO₂ - Molar mass of CO₂ = 44.01 g/mol.
- Moles of CO₂ = \( \frac{1.467 \, \text{g}}{44.01 \, \text{g/mol}} = 0.0333 \, \text{mol} \).
- Since each mole of CO₂ has 1 mole of C, moles of C = 0.0333 mol.
- Mass of C = \( 0.0333 \, \text{mol} \times 12.01 \, \text{g/mol} = 0.400 \, \text{g} \).
3. Determine the Mass of Hydrogen from H₂O - Molar mass of H₂O = 18.02 g/mol.
- Moles of H₂O = \( \frac{0.600 \, \text{g}}{18.02 \, \text{g/mol}} = 0.0333 \, \text{mol} \).
- Each mole of H₂O has 2 moles of H, so moles of H = \( 0.0333 \times 2 = 0.0666 \, \text{mol} \).
- Mass of H = \( 0.0666 \, \text{mol} \times 1.01 \, \text{g/mol} = 0.0673 \, \text{g} \).
4. Determine Mass of Oxygen (if present) Since the compound is a hydrocarbon (only C and H), we skip this step. If oxygen were present, we would subtract the masses of C and H from the initial sample mass to find the mass of O.
5. Calculate Moles and Find Empirical Formula - Moles of C = 0.0333 mol; Moles of H = 0.0666 mol.
- Divide by the smallest number of moles (0.0333) to get the ratio:
    - Carbon: \( 0.0333 / 0.0333 = 1 \)
    - Hydrogen: \( 0.0666 / 0.0333 = 2 \)
- The empirical formula is CH₂.
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