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Chemistry - calculating equilibrium constant, what am I doing wrong?

  1. Feb 12, 2006 #1
    I feel like I know what I'm doing but I'm not getting an answer that is offered. Heres the question:

    Dinitrogentetraoxide partially decomposes according to the following equilibrium:

    N2O4 (g) ---> 2NO2 (g) (an arrow going other way too)

    A 1.00-L flask is charged with .400 mol of N2O4. At equilibrium at 373 K, 0.0055 mol of N2O4 remains. Equil. constant for this reaction is __________.

    a. 2.2 x 10^-4
    b. 13
    c. 0.22
    d. 0.022
    e. 0.87

    K = [NO2]^2 / [N204]

    N2O4 goes from .4 to .0055 mol, so that is a change of .3945 mol, which must be the equilibrium amount of the 2NO2. Since the volume is just 1 L

    K = [.3945 M]^2 / [.0055 M] = 28.3

    but this is not one of the choices.

    what am I doing wrong?
    Last edited: Feb 12, 2006
  2. jcsd
  3. Feb 12, 2006 #2


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    First thing you're "doing wrong" is attempting to reproduce the errors in the instructional software; you already came up with an answer of one hundred something, didn't you. You wouldn't be stating, "... .3945 mol, which must be the equilibrium amount of the 2NO2," otherwise.

    If you know what you're doing, it's your call how you handle it; try to reproduce the software errors (and not rock the boat), or present your work to the instructor (or go to the department head) and get a few TAs into trouble.
  4. Feb 12, 2006 #3


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    No, there is no law of conservation of moles !

    If you use up 1 mol of N2O4, you make 2 moles of NO2. In any case, the correct answer still does not exist in the provided list. Have you copied the question correctly ? Or have you mistyped choice (b) ?
  5. Feb 12, 2006 #4
    Hi, yes the answers are exactly as my WEBCT has them.

    I understand where I was messing up on the moles, but now I have an answer of 113!

    Relate the change
    .3945 mol N2O4/L (2 mol NO2/1 mol N2O4) = .789 mol NO2/L

    0+.789 = .789 for the NO2 so [.789]^2/[.0055] = 113

    is that the answer that should be correct? the 13 was just typed in incorrectly and should have been put in as 113 into webct??
  6. Feb 12, 2006 #5


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    Yes, that would be my guess. Take this to your TA or instructor.
  7. Dec 14, 2008 #6
    Equilibrium constant Help!

    How do we calculate equilibrium constant if we do not know what the concentration is and all else is given in the formula?
  8. Dec 15, 2008 #7


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    Staff: Mentor

    Please elaborate (and don't hijack the threads).
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