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Mixture of acetone and CS2

  1. May 10, 2014 #1

    utkarshakash

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    1. The problem statement, all variables and given/known data
    Acetone and carbon disulphide form binary liquid solution showing positive deviation from Raoult's law, The normal boiling point(BP) of pure acetone is less than that of pure carbon disulphide. Select the incorrect statement(s) among the following.

    A)BP of mixture is always less than the BP of acetone.
    B)BP of the azeotropic mixture is always less than the BP of pure CS2.
    C)When a small amount of CS2 is added to an excess of acetone, BP of solution increases.
    D)A mixture of acetone and CS2 can be completely separated by simple fractional distillation.


    3. The attempt at a solution

    The first option is incorrect as BP always increases. Second option is correct as the mixture is a minimum BP azeotrope. I'm confused about the third option. The answer says it is incorrect but according to what I've read, BP always increases on addition of solute. For option D, I'm not sure how to think about that as I don't know much about fractional distillation and how it can be applied to this problem.
     
  2. jcsd
  3. May 10, 2014 #2

    AGNuke

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    C) What happens to the vapor pressure of the solution when you do that? Can you derive the impact of that on BP of the solution?

    D) Well, you might want to check the BP of individual components before answering that.
     
  4. May 10, 2014 #3

    utkarshakash

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    C) The vapor pressure increases so the BP should decrease. I guess then CS2 is not considered to be a solute in this case, right?
     
  5. May 10, 2014 #4

    utkarshakash

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    Do I need to know the exact numerical values?
     
  6. May 10, 2014 #5

    AGNuke

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    Nah. Just a rough idea. If they are close, then simple fractionating column might not cut it. You can learn more about fractional distillation if you want. But if I were you, I might have went with the option D.

    And regarding to the C option, I suppose what you said is applicable more appropriately with "non-volatile" solute. Since our solute is volatile, it affects the solution in a rather interesting way (i.e. positive deviation from raoult's law)
     
  7. May 10, 2014 #6

    utkarshakash

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    It all makes sense to me now. Thanks!
     
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