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Bpt elevation, why?

  1. May 11, 2008 #1
    Hullo. I'm doing a lab report on boiling point elevation, and we have to answer the question: "the boiling point elevation is higher than the calculated value, Why?". as a question on the side.

    In the experiment, the solution is 30g/l NaCl in water.
    The equation used to calculate the calculated value is Boiling point elevation = Molal elevation const. x Molality.
    Molal elevation const = 0.52

    I'm looking for some kind of molecular description, I thought it might be to do with breaking ligand bonds between the water and the salt during evapouration, but dissolving NaCl is endothermic (according to wikipedia~~~), plus i don't think it would have that effect until you boil it dry.

    I know the equation assumes it's an ideal solution, and it's not cos NaCl is ionic, what can't explain is why the boiling point elevation would be more than the calculated value, rather than just different from it...

    any help greatly apprecaited, thanks.
     
    Last edited: May 11, 2008
  2. jcsd
  3. May 11, 2008 #2
    Wht happens to the vapour pressure when NaCl is added to water?Does it increase or decrease?Then remember tht the definition of the normal boiling point is where the vapour pressure equals 1 atm.
    Btw this is the wrong place to ask such questions...this is a chemistry question
     
  4. May 11, 2008 #3

    Borek

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    Dissociation taken into account?

    Boiling point elevation is taught at chemistry lessons, but I have problems classifying it as a just a chemistry subject.
     
  5. May 11, 2008 #4

    Sorry, i posted it in a rush. Can I move it myself, Or does a moderator have to do it?
     
  6. May 11, 2008 #5

    Borek

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    Don't worry, if one of Mentors will decide to, they'll move the thread.

    Have you taken dissociation into account?
     
  7. May 11, 2008 #6
    I'm not really sure what you mean, I know what dissociation is I know the salt forms Na+ and Cl- in water, but there's nothing about that in the equation...
     
  8. May 11, 2008 #7
    The equation of osmotic pressure is equal to iMRT....i is the vant hoff factor i.e. the number of particles it dissociates into.
     
  9. May 12, 2008 #8
    Sorry to bump my own thread, but this report has to be in for 2 tommorow, and i've got lectures all day, so it is a matter of some urgency.

     
  10. May 12, 2008 #9

    Borek

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    Staff: Mentor

    Everything you need was already listed in terminator88 and my posts.
     
  11. May 13, 2008 #10
    k it seeems u r not reading wht I have said....
     
  12. May 13, 2008 #11
    ach, It doesn't matter, I handed in the report today anyway. Thanks anyway.
     
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