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Homework Help: Getting Empirical Forumula Using Ideal Gas Laws

  1. Sep 27, 2009 #1
    1. The problem statement, all variables and given/known data
    CxHy gets burned in oxygen gas and makes CO2 and water. Given the partial pressure of water and the total pressure of the resulting mixture I need to find the empirical formula.

    2. Relevant equations
    Well PV=nRT is the obvious choice...but all the information you see there is all that's given. The only other thing I was able to find was the pressure of the CO2 just doing some subtraction. The problem doesn't state a volume, temperature or anything else.

    Can I assume that temperature is a constant and eliminate it? How about volume? I'm not sure about that, if I can then it might help.
  2. jcsd
  3. Sep 27, 2009 #2


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    Homework Helper
    Education Advisor
    Gold Member

    The problem description is far too general. You must have some known given conditions.
  4. Sep 27, 2009 #3
    Nope that's it. All we know is that a hydrocarbon was burned, and the pressure of the products. We can't assume standard conditions because the pressure is off.

    There's another problem I couldn't solve:

    A mixture of the gases CO and CH3COCH3 (acetone) is trapped in a 1.0 L flask. The pressure in
    the flask registers 100 mmHg initially but the pressure registers 114 mmHg after the acetone in the
    flask is caused to decompose according to the following reaction at the same temperature:
    CH3COCH3(g) H C2H4(g) + CO(g) + H2(g)
    If all the substances present are in the gas phase, and CO is unchanged by any chemical reaction,
    what were the inital and final pressures of CO, assuming complete reaction of the acetone?

    Again, how can I proceed? I don't think enough is known.
  5. Sep 27, 2009 #4
    Well you didn't include the pressure in your posting. :wink:
  6. Sep 28, 2009 #5

    The total pressure of the mixture is 1.2 atm and the pressure of the H2O is .686 atm. That still doesn't help much...does it?
  7. Sep 28, 2009 #6


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    Staff: Mentor

    Yes it does - it gives you partial pressure of CO2, and that means you know exact molar ratio of H2O and CO2.

  8. Sep 28, 2009 #7
    Actually it's just carbon monoxide. So that means everything in the decomposition is 1:1. I'll canker around with it and see where I can get. Dalton's pressure law seems a reasonable place to start.
  9. Sep 28, 2009 #8


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    Staff: Mentor

    Sorry, misread the question - but it doesn't matter much, it is still molar ratio that counts.

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