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Topic: Where am I going wrong with this question? Grade 11 Organic chem. Exam review.  (Read 1700 times)

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Offline a2242364

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The question is this: If 129 g of oxygen gas and 300 g of propane are mixed and allowed to react, determine the volume of water vapour formed at 116 kPa and 120 degrees celcius.


So I made the chemical equation: C3H8 + 5O2 -> 3CO2 + 4H20


Without showing steps here, I found that oxygen is the limiting reagent, and used that to find the number of moles of water vapour to be 4.03 mol. After this, I used the Ideal Gas Law to calculate the volume of water vapour.

PV = nRT
V = nRT / P
V = (4.03*8.314*393.15) / 116
V = 113.56 L

This is wrong, and supposedly the correct answer is 91.0 L. Can anyone be so kind to point out where I went wrong? Did I make a big mistake somewhere or did I just mess up calculations some how (I doubt it because I spent a good amount of time looking the question over, but who knows lol).

Thanks so much in advance.

Offline AWK

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4.03 is to much.
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Offline Enthalpy

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This chemical equation would hold only with an excess of oxygen and with something (usually a lot of nitrogen) that limits the temperature and at a reasonable pressure.

Here I agree that the reaction lacks oxygen. This does NOT mean that only a part of the propane is burnt and this part follows the equation. With such an oxygen/fuel mismatch, expect all the propane to be affected, but not form these products, or not only these.

So among the products, you may also have H2, CO, C. The temperature must tell you which ones you can have, which one takes the oxygen to the others, and then the available oxygen tells the amount possible for each product.

Or you do it with chemical equilibria CO/CO2, H2/H2O, and so on - or between the elements and the possible products. But at 120°C the "equilibria" will mean: everything for one, nothing for the other.

The question implicitly suggests that the products are at equilibrium, and I dislike that. Don't ask too many question to get a good mark, but please keep in mind for real life that a combustion is not always at equilibrium. For instance, once soot is formed, it won't burn even if you add oxygen, even at high temperature.

Offline AWK

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Enthalpy is absolutely right, but his objections should be adressed to the author of this problem (or the author of textbook), not to the poor student starting chemical calculations.
Moreover, only at a very large excess of hydrocarbon, eventual amount of hydrogen during partial combustion is important (I agree with C and CO). In this case the question concerns only water vapours. Hence the problem can be solved by correct stoichiometry of combustion reaction and using ideal gas law.
a2242364 calculated only moles of oxygen which is not equal to moles of water.
« Last Edit: June 12, 2016, 04:53:40 PM by AWK »
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