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Homework Help: Pressure and concentration equilibria

  1. Apr 20, 2009 #1
    1. COCl2(g) D CO(g) + Cl2(g) Kp = 6.8 x 10-9 at 100oC

    What will be the equilibrium concentrations of all 3 gases if 1.0 atm of phosgene (COCl2) is placed into 5.0L reaction vessel?

    The answer is

    [COCl2]eq = 0.033M
    [CO]eq = [Cl2]eq = 2.69 x 10-10M

    I don't get how the second one is solved. I used this method

    I found out it's going towards the products because the reactant is decomposing and there is no products yet to be in equilibrium

    Finding initial 1 -> 0 + 0
    Finding the amount taken out -x +x +x

    I can use the assumption method to assume that since Kp is sooo small x can be ignored for 1-x so....

    Kp= x^2/1

    Also since it can be ignored i assumed that the equilibrium pressure for phosgene is 1 and then i used C=P/RT to get 0.033.

    However! for the concentrations of the products I can't seem to get them at all!!

    I tried converting Kp to Kc --> Kc=Kp(RT)^-delta (n)

    delta(n)=2-1=1 so its Kc= Kp/RT which equals 2.27e-10.

    So I tried doing products/reactants=Kc

    but (2.69 x 10-10)^2/0.033 does NOT equal 2.27e-10 so I'm lost to where I went wrong with this equation...
  2. jcsd
  3. Apr 22, 2009 #2


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    Gold Member

    So, in the denominator you think that the 'x' (1-x in your analysis) is small enough to be ignored but you want to assume it is there in the numerator. I'm afraid you are going to have to do a complete solution. 'Complete' means that 1-x is not equal to 1.

    You can make that assumption (x=0) when you are only using two significant figures to denote the phoshene concentration but you can't if you ever want to actually calculate 'x'.
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