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Homework Help: Determine the partial pressure of gas at equilibrium

  1. Oct 19, 2012 #1
    I figured out the answer to this already, but I wanted help on the reasoning behind it:
    ScreenHunter_05 Oct. 19 17.29.jpg

    In order to work out the problem we're supposed to determine the partial pressure of phosgene by subtracting 0.497 atm worth from the initial pressure of 1.31 atm (determined by using the ideal gas equation). My question is... why? I mean, if there's 0.497 atm of CO(g) and the stoichiometric ratios of the balanced equation are 1:1:1, then why aren't all three gases at 0.497 atm?
    Last edited: Oct 19, 2012
  2. jcsd
  3. Oct 19, 2012 #2
    Because the stoichiometric ratios are not 1:1:1.

    If they were, the equilibrium constant would be equal to 1/(partial pressure of any of the three gases) -- and that could not be a constant!

    You need to think differently about what the meaning of the stoichiometric equation might be.
  4. Oct 19, 2012 #3
    If the balanced equation is [itex]CO_{(g)} + Cl_{2(g)} ⇋ COCl_{2(g)}[/itex], then how is the relationship not 1:1:1? The equation outright says that there's 1 mol of COCl2, 1 mol of CO, and 1 mol of Cl2.
  5. Oct 19, 2012 #4
    The next question is basically the same type of question, too:
    ScreenHunter_06 Oct. 19 19.14.jpg

    The first part of the answer is:
    ScreenHunter_07 Oct. 19 19.14.jpg

    So, I'm confused about the same thing here, essentially (well... very similar, at least). In the answer, the equilibrium pressures add up to 117% (0.66+0.34+0.17=1.17). I don't understand how that's possible.

    If 34% of the initial gas dissociates, then 66% is still NOBr (that much we seem to agree on, at least). Doesn't that mean that 34% will be a combination of 1 part NO and 1/2 part of Br2? (because we're using percent compositions here) I'd think that if 66% is NOBr, then 22.666% would be NO, and 11.333% would be Br2.
  6. Oct 20, 2012 #5
    If you start out with y mol of phosgene, then what the balanced equation tells you is that when x mol of it react to produce carbon monoxide and chlorine, the number of mole of the three substances will be y – x , x , and x respectively. That is not a 1:1:1 ratio!
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