To determine the empirical formula from a combustion reaction, you need to analyze the products of the reaction, specifically carbon dioxide (CO2) and water (H2O), and follow a step-by-step process to calculate the mole ratio of each element in the original compound.
Here is a comprehensive guide:
Steps to Find Empirical Formula from Combustion Data
1. Identify the Masses of Products
- Step 1: Begin by noting the mass of carbon dioxide (CO2) and the mass of water (H2O) produced during the combustion. These values will be provided in the problem statement or determined through experimentation.
2. Calculate Moles of Carbon
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Step 2: Use the mass of carbon dioxide (CO2) produced to calculate the moles of carbon.
- Recognize that each molecule of CO2 contains one atom of carbon.
- Convert the mass of CO2 to moles using its molar mass (approximately 44.01 g/mol).
- The moles of carbon in the original compound are equivalent to the moles of CO2 produced.
Formula: Moles of Carbon = (Mass of CO2 / Molar Mass of CO2)
3. Calculate Moles of Hydrogen
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Step 3: Use the mass of water (H2O) produced to calculate the moles of hydrogen.
- Recognize that each molecule of H2O contains two atoms of hydrogen.
- Convert the mass of H2O to moles using its molar mass (approximately 18.02 g/mol).
- Multiply the moles of H2O by two to get the moles of hydrogen in the original compound.
Formula: Moles of Hydrogen = 2 (Mass of H2O / Molar Mass of H2O)*
4. (If Applicable) Calculate Moles of Oxygen
- If the compound contains oxygen, the moles of oxygen must be determined. This is usually done by subtracting the mass of carbon and hydrogen from the total mass of the original compound.
5. Determine Mole Ratios
- Divide the number of moles of each element by the smallest number of moles calculated. This will give you the simplest mole ratio, which will be the subscripts of the empirical formula.
6. Write the Empirical Formula
- Write the chemical formula using the mole ratios calculated.
Example Calculation
Let's say that the combustion of a hydrocarbon produces 88.02 grams of CO2 and 36.04 grams of H2O.
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Step 1: We have 88.02 g of CO2 and 36.04 g of H2O.
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Step 2: Moles of C = (88.02 g / 44.01 g/mol) = 2 moles.
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Step 3: Moles of H = 2 * (36.04 g / 18.02 g/mol) = 4 moles.
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Step 4 (not applicable in this hydrocarbon example): There are no other elements to consider for this example.
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Step 5: Mole ratio of C:H = 2:4. Dividing by 2 to get the simplest whole number ratio, we get 1:2.
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Step 6: The empirical formula is CH2.
Practical Insights
- Accuracy: Precise measurements of the masses of CO2 and H2O are crucial for obtaining an accurate empirical formula.
- Assumptions: These calculations assume that all carbon in the original compound converts to CO2 and all hydrogen converts to H2O.
- Oxygen Content: When oxygen is in the original compound you must calculate this by subtracting the carbon and hydrogen from the total mass of the original compound. This may require extra steps of calculation.
By systematically following these steps, you can accurately determine the empirical formula of a compound using combustion reaction data.
