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Vapor Pressure of saturated/unsaturated solution

  1. Aug 19, 2015 #1
    1. The problem statement, all variables and given/known data:

    As the picture shows, there are two NaClaq solutions. One is saturated while the other is not and both of them are at the same temperature. There are three questions asked:
    a) Above which solution is the vapor pressure of water, PH2O, greater? Explain.
    b) Above one of these solutions, the vapor pressure of water, PH2O, remains constant, even as water evaporates from solution. Which solution is this? Explain.
    c) Which of these solutions has the higher boiling point? Explain.


    2. Relevant equations

    As Raoult's law states: Psolution=Psolvent*Xsolvent
    where Xsolvent is the mole fraction of the solvent in the solution. In this case, Xsolvent is XH2O.


    3. The attempt at a solution

    **For question a), solution (2) has a greater PH2O value because the amount of solute (in this case NaCl) is less than that of solution (1) therefore Xsolvent of solution (2) is greater than that of solution (1).

    **For question c), solution (1) has a smaller PH2O value and therefore the boiling point of solution (1) would be higher than that of solution (2).


    I'm left confused by question b) because I cannot prove how PH2O could remain constant. I would be glad if someone could explain the logic of this question. Please check as well if the previous answers/explanations for question a) and c) are appropriate or not.
     

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    Last edited: Aug 19, 2015
  2. jcsd
  3. Aug 19, 2015 #2
    I've made a mistake for question a) & c)


    For question a), solution (2) has a greater PH2O value because the amount of solute (in this case NaCl) is less than that of solution (1) therefore Xsolvent of solution (2) is greater than that of solution (1).


    For question c), solution (1) has a smaller PH2Ovalue and therefore the boiling point of solution (1) would be higher than that of solution (2).
     
  4. Aug 19, 2015 #3

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    What happens to the solutions as water evaporates? Describe both cases.
     
  5. Aug 19, 2015 #4
    As water evaporates, the amount of solvent (water) in both solutions decreases. For solution (2) the mole fraction of water should decrease in value. If I'm not mistaken, for solution (1) (which is saturated), as the water evaporates, some of the salt solidifies as well in proportion. So, the mole fraction of solvent of solution (1) remains constant.

    Is this correct?
     
  6. Aug 19, 2015 #5

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    Very good.
     
  7. Aug 19, 2015 #6
    So the answer is definitely 2,1,1?
     
  8. Aug 19, 2015 #7

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    "Not saturated, saturated, saturated."
     
  9. Aug 19, 2015 #8
    Thanks a bunch!
     
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