A sample of a hydrocarbon is combusted completely in oxygen(gas) to produce 21.83 grams of carbon dioxide (gas) and 4.47 grams of water (gas) and 311kJ of heat. What is the mass of the hydrocarbon sample that was combusted? What is the empirical formula of the hydrocarbon? Calculate the value of heat of formation per empirical formula unit of the hydrocarbon?

This is actually very common in chemistry -- and very easy.

The equation for combustion of a hydrocarbon is



You have 21.83 grams of CO2 and 4.47 grams of water.

1. Determine the number of moles of CO2 that were produced (0.496)
2. Determine the number of moles of H2O that were produced (0.248)
3. From m and n, you can determine the empirical formula of CnHm. Using the equation given above, you can determine fractions for the values of m and n. They might be something like m=0.443 and n = 0.22. The usual exercise is to divide both numbers by the smaller to get a 1 for the smaller number and another number for the larger one. 0.22/0.22 = 1; 0.443/0.22 = 2.01 So, the empirical formula in my example would be CH2. You might come out with something like 1:1.5. You'd then know that the empirical formula would have to be C2H3.

For the second question, you know the amount of heat that's produced, so you work out the amount of heat per "formula weight" using the formula that you determined.

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Originally Posted by Perito

This is actually very common in chemistry -- and very easy.

The equation for combustion of a hydrocarbon is



You have 21.83 grams of CO2 and 4.47 grams of water.

1. Determine the number of moles of CO2 that were produced (0.496)
2. Determine the number of moles of H2O that were produced (0.248)
3. From m and n, you can determine the empirical formula of CnHm. Using the equation given above, you can determine fractions for the values of m and n. They might be something like m=0.443 and n = 0.22. The usual exercise is to divide both numbers by the smaller to get a 1 for the smaller number and another number for the larger one. 0.22/0.22 = 1; 0.443/0.22 = 2.01 So, the empirical formula in my example would be CH2. You might come out with something like 1:1.5. You'd then know that the empirical formula would have to be C2H3.

For the second question, you know the amount of heat that's produced, so you work out the amount of heat per "formula weight" using the formula that you determined.

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How do you calculate the mass of the hydrocarbon?

Once you know the "formula" CnHm (you have calculated n and m), you figure out the number of moles of CnHm that burned (1 mole per n CO2 or 1 mole per m/2 H2O) and multiply that by the formula weight of CnHm.