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Homework Help: Chemical Equilibria with Gases

  1. Mar 19, 2012 #1
    1. The problem statement, all variables and given/known data

    When 1.000 mol PCl5 is intorduced into a 5.000L container @500K, 78.50% of PCl5 dissociates to give equilibrium mixture PCl5, PCl3, Cl2 : PCl5--->PCl3+Cl2 (this eq. is reversible)

    (a) calculate Kc and Kp
    (b) if the initial concentrations are [PCl5]=.500M, [PCl3]=.150M, and [Cl2=.600], in which direction does the rx proceed to reach equilibrium? what are the concentrations when the mixture reaches equilibrium?

    2. Relevant equations


    3. The attempt at a solution

    I attempted to find Kc of the initial problem, by getting the molarity (.2) of the reactant. I then multiplied by .785 to get the percent to put in the numerator, but am not getting the right answer in the book. I know in part b i must use the chart to convert to quadratic equation, but am having trouble with the first part (surprisingly, as its usually the second part which gives me trouble). I have tried this either which way and still can not get the right answer. Any input would be great. Thanks!
  2. jcsd
  3. Mar 19, 2012 #2


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    Staff: Mentor

    Not seeing all the numbers it is hard to say - you can be missing something, it can be just a math error. Please show your complete work, step by step.
  4. Mar 20, 2012 #3
    Yeah turns out I had the right idea, but didn't do it right. I went and saw my professor and he walked me through it. I had assumed that the .2M stayed the same for the PCl5, however, when the equation went to equilibrium, I had forgotten to use the 21.5% that was left, as the PCl5.

    .2M *.785 gives .157, then .2-.157 gives .043, which is what I should have used for the PCl5 @ equilibrium, rather than the .2. Thanks for the input. Glad to know that people actually respond and are willing to help out.
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