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Homework Help: Calculating equilibrium constant

  1. Feb 23, 2014 #1
    1. The problem statement, all variables and given/known data

    Estimate the value of the equilibrium constant at 670K for the following reaction: 2NO2(g)⇌N2O4(g)

    2. Relevant equations

    ΔG°rxn = -RT*lnK

    3. The attempt at a solution

    This should be an easy problem. But for some reason, MasteringChem won't accept it, so I'm wondering if I did anything wrong. Here's what I did:

    1) Using tabulated free energies of formation, I obtained ΔG°rxn=-2.8 kJ=-2.8x10^3 J.
    2) I then divided this by 8.314 J/molK multiplied by 670K (multiplied by -1, which cancels).
    3) I took e^ of both sides. K=1.65

    Anyone know what I'm doing wrong? Just a small hint will suffice. Thank you in advance :)
  2. jcsd
  3. Feb 24, 2014 #2


    User Avatar

    Staff: Mentor

    I get the same answer.

    I would've used 8.315 J K-1 mol-1, but I don't think that's the problem here.

    This is where things go bad! Could you show your calculation?
  4. Feb 24, 2014 #3
    In your table, what temperature is that ΔGo at? I bet it's not at 670 K.

  5. Mar 2, 2014 #4
    @ DrClaude and Chestermiller:

    Sorry for the late response. Thank you for reviewing my post. To DrClaude: I forgot to account for the temperature in my calculations, as noted by Chestermiller. I calculated the std. change in enthalpy and entropy, and used the values to find ΔG^o at 670K. Then I plugged it into the equation from my first post to get a new answer of K=1.34e-5 (which turned out to be correct). Thank you both once again for looking at my post, and Chestermiller for offering that helpful bit of advice. :)
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