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chemistry practise set 6 #3
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Brittany_6462
#1 Posted : Wednesday, May 19, 2021 11:18:08 PM
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Can someone please explain where 23.5 moles came from? The combustion equation in the passage says invalid equations so not sure if that is why I didnt understand the calculations. I thought we divide the 167g by the molar mass of C12H23 and then do stoichiometry to find the moles of gas produced. then plug that into pv=nRT
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Youssef_6546
#2 Posted : Thursday, May 20, 2021 5:23:31 AM
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Hi Brittany,

Yes, I agree that getting to see that combustion reaction under “invalid equations” would have helped in answering this question. However, recall from high school chemistry and/or first/second year intro to university chem class that combustion of hydrocarbons usually involve the hydrocarbon and O2 as reactants, then CO2 and H2O as products. So, pretty much the combustion of C12H23 would have this balanced equation:

4 C12H23 + 71 O2 —> 48 CO2 + 46 H2O

How I approached this is I divided both reactant and product side by 4, as we want to determine the number of moles of gas are produced (CO2 and H2O gas) from one mole of C12H23. In this case, we’ll get 12 mol CO2 and 11.5 mol H2O, which add to 23.5 moles. We then plug this value into PV=nRT to get V. In this case, our answer should be approximately 3400 L (Option D)

Hope these help!
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INSTR_Katrina_128
#3 Posted : Thursday, May 20, 2021 10:27:30 PM
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Hi all,

I've let Head Office know about the "Invalid Equation" image. Thanks for spotting that Brittany.

Great analysis again, Youssef!

The main takeaway from this is that you need to determine the following:

For every 1 mole of fuel (C12H23), how many moles of product gas do we produce? In this case, it is 12 + 11.5 = 23.5 moles.

Why do we not differentiate between the two species of gas? Well, that's because we are using the ideal gas law in the end, and the ideal gas law doesn't CARE about the species that you have, AS LONG AS THEY ARE GASES! So we can lump both sets of gases (water and CO2) together and calculate the volume of product gas all at once!
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