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General Chemistry Problem help

Discussion in 'DAT Discussions' started by Nasem, Apr 13, 2007.

  1. Nasem

    Nasem 2+ Year Member

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    Aug 30, 2006
    Lansing, MI
    An ideal solution of toluene (vapor pressure 21 torr) and cyclohexane (vapor pressure 67 torr) in which the mole fraction of cyclohexane is 0.74 is boiled and the vapor condensed to a liquid. What is the mole fraction of cyclohexane in the condensed liquid?


    I can't figure this darn thing out... I always thought that the mole fraction of a solution (regarding phase) is the same no matter what... So I thought the answer would be 0.74 and its not....

    Can anyone help plz
     
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  3. DentalKitty

    DentalKitty 5+ Year Member

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    Mar 12, 2007
    The only thing I can think of is that they heated it at a temp high enough to boil the cyclohexane, but not high enough to boil the toluene (about 20 deg C difference in bp's). This would make the vapor have a higher concentration of cyclohexane than in the original solution. As for how to get a mathematical answer from that, I am not sure. Unless the answer is 1.0 cyclohexane (which I'm sure it's not). If anyone knows, please help us out :)
     
  4. poc91nc

    poc91nc Banned Banned 7+ Year Member

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    Nov 5, 2006
    We are given the partial pressures for both compounds. So, the total pressure above the solution must be 21 + 67. Given the information can't we just use the equation for partial pressure?

    88torr(mol fraction of toulene) = 21 torr = partial pressure of toluene

    21/88 = mole fraction of toluene in the vapor above the solution. That's how I would do it...
     
  5. poc91nc

    poc91nc Banned Banned 7+ Year Member

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    Nov 5, 2006
    whoops...you need partial pressure for cyclohexane...just do the opposite then.
     
  6. poc91nc

    poc91nc Banned Banned 7+ Year Member

    552
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    Nov 5, 2006
    total pressure = 88
    partial pressure of cyclohexane 67

    88(mole fraction of cyclohexane in vapor) = 67

    67/88
     
  7. poc91nc

    poc91nc Banned Banned 7+ Year Member

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    Nov 5, 2006
    Just read the question again. Actually, we have to calculate the partial pressures first...thought they were given.
    67(0.74) = 49.5
    21(1 - 0.74) = 20.3
    total pressure = 69.8
    69.8 (mole fraction of cyclohexane) = 49.5
     
  8. poc91nc

    poc91nc Banned Banned 7+ Year Member

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    Nov 5, 2006
    Damn...the second partial pressure is 5.46....put the wrong number in the calculator. So....total pressure is 52.8
     
  9. poc91nc

    poc91nc Banned Banned 7+ Year Member

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    Nov 5, 2006
    uhghh...nevermind...it's 54.96. I haven't used a calculator in so long (on purpose) I think I forgot how to use it. But yeah...there is a similar problem in one of the Achiever tests and that is how you solve it (but I think they gave you the partial pressures). Calculating them seperately would be too time consuming for a practice test...and the numbers aren't very friendly.
     

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