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vapor pressure

Discussion in 'DAT Discussions' started by ippie, Aug 2, 2006.

  1. ippie

    ippie ippie
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    I am always confused with vapor pressure.
    The vapor pressure of the mixed solution is lower than the pure solvent.(Roult's law).
    The lower vapor pressure means more easily vaporized or more hardly vaporized?
     
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  3. lintydent

    lintydent member
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    In the context of being able to vaporize, gas-liquid...vapor pressure is the amount of gas pressure that is exerted over the liquid pressure. If vp increases that means there is more gas pressure exerting over the liquid pressure, which means that the liquid can vaporize more easily. If the vapor pressure is low, there is less gas pressure exerted over the liquid, and thus less easily vaporized ("more hardly vaporized").
     
  4. ippie

    ippie ippie
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    Then, how can I solve this problem?

    According to Raoult’s law,

    A. positive deviation occurs when partial pressures of molecules from a mixed liquid are lower than predicted.
    B. negative deviation occurs when partial pressures of molecules from a mixed liquid are higher than predicted.
    C. positive deviation occurs when molecules from a mixed liquid escape more difficultly than their pure counterparts.
    D. positive deviation occurs when molecules from a mixed liquid escape more easily than their pure counterparts.
    E. negative deviation occurs when molecules from a mixed liquid escape more easily than their pure counterparts.

    The answer is D.
    According to your theory, the vapor pressure of mixed solution is lower than pure solvent. --> lower vapor pressure means "hardly vaporized"
    I usually think the same way, but how can I explain above problem?
     
  5. lintydent

    lintydent member
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    To help understand, you would need to define +/- deviation...which basically means if there is more vapor pressure than expected it is increasing from where it is and deviates positively, or if there is less vapor pressure than expected, from where it is, then it is deviating negatively.

    So since we established that mixes solutions have a LOWER vp than pure solutions:

    A. when mixed solutions are lower than expected, they are decreasing and deviating more negativley than expected = (-) deviation, not (+)
    B. when mixed solutions are higher than expected = (+) deviation, not (-)
    C. when mixed solution is more difficult than it already is, it's becoming more negative = (-) deviation, not (+)
    D. when mixed solution is escaping more easily than the pure, it's becoming more easily or more positive than it should = (+) deviation
    E. same as D. ...not (-) but (+)

    Hope this makes sense!
     

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