Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: methic on October 17, 2010, 01:02:49 PM
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I have no idea how to start this problem. Well, I started it; but I don't know if I started it correctly.
A gaseous fuel mixture stored at 744 mmHg and 298 K contains only methane, CH4, and propane, C3H8. When 12.8 L of this mixture is burned, it produces 776 kJ of heat.
What is the mole fraction of methane in the mixture."
So, this is what I've done so far. I found the total number of moles burned through PV=nRT
(0.98atm)(12.8L)=n(0.08206)(298K) = 0.512 moles of mixture.
I'm not sure if I need these...
Molar mass of CH4 is 16.04 g/mol
Molar mass of C3H8 is 44.10 g/mol
I'm basically stuck there. I have no idea how to set up the problem. ???
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You will need standard enthalpies of burning for both gases.
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Hi Borek. Thanks for replying.
So, I got them. Methane at -890.3 and Propane at -2219.1. Is it just a matter of solving for x in an equation like this:
-890.3x + -2219.1y = -776 kJ
And that will give me the moles of x, at which point I use the total moles I previously found to get the mole fraction?
y being (0.512-x),
so -890.3x + (-2219.1)(.512-x) = -776kJ
x = .27 mol / .512total moles = 53% CH4
Is that correct?
//edit: Incorrect. Got it wrong. :( 'supposed to be 42.7%. Don't know where that came from
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Your approach is correct - as far as I can tell there is nothing more to the question. Could be there is some problem with the enthalpies you used.