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Freezing Pt Depression / Boiling Pt. Elevation............equations.

Discussion in 'DAT Discussions' started by DRHOYA, Jun 19, 2008.

  1. DRHOYA

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    Hey guys. I was doing questions in the G-chem section of destroyer today, and noticed something. In destroyer, there were a few simple problems relating to freezing point depression or boiling point elevation, and usually asking to solve for one of the terms, or mw from the molality. Anyway, I totally understand the chemistry behind these problems, but my question lies within the equations used.

    Destroyer was using the equation (ie: for bp) Tbp = (Kbp)(m)(i), where (i) is the "Vant-Hoff" factor (a coefficient factored in for species that can dissociate, for ex: Na2(NO3) can break up into Na and NO3, so the (i) = 2). But in KAPLAN, they do not use (i) in their equation. Now, I know when (i) = 1 it doesnt matter............but the few times you do need it...which equation is correct???

    Also, the same question but for a different equation. Involving osmoregulation n = MRT(i).....again destroyer uses the (i), and KAPLAN does not.

    Thanks in advance. Anyone shed some insight?
     
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  3. osimsDDS

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    Yea so first of all vant hoff factor is VERY IMPORTANT, it can mean you get the answer right or wrong...there is an easy way to use it and understand it for colligative properties (properties of the dissociated ions only remember!!!) ok so basically when you have NaCl for example it can dissociate into 2 moles...Na+ and Cl- in solution

    Since were talking about colligative properties this holds true for all colligative property equations, osmotic pressure, bp elevation, fp depression..etc

    Sugars have a Vant-Hoff factor of 1, and also sometimes they tell you in the question non-dissociating compound or non-electrolyte...

    hope that helps
     
  4. Bruinlove

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    This is a mistake by Kaplan that I caught while taking topscore exam.

    ' i ' should be multiplied when there is dissociation.
     
  5. osimsDDS

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    Also I think Na2NO3 can split up into 3 moles in solution...2 Na and 1 NO3^2- so the vant-hoff is actually 3 but im not sure...

    Here is an easy way to check just write out the dissociation of the compound:

    Na2NO3 ----> 2Na+ + NO3^2-

    Therefore, you started with 1 mole, and it dissociated into 3 (2 Na and 1 NO32-)

    Usually to get van-hoff factor you do the mole ratio of products over reactants...

    In this example it would be 3/1 = 3 is vant-hoff factor
     
  6. harrygt

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    Hey Buddy, I don't have destroyer, but I have noticed the weakness of Kaplan in that section before, as you did.
    You always have to include the Van Hoff factor in the equation if the particle dissociates in the solution. Kaplan has simply skipped this important and tricky point. Big Weakness!!!
    You gotta include the Van Hoff factor in Pi = iMRT too. Just as you mentioned. Shame on Kaplan!
     
  7. harrygt

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    Hey Osim, there is no Na2NO3. It is NaNO3, which has a Van Hoff factor of 2. I think the 2 appeared there just as a typo.
     
  8. Simiam

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    It is a mistake in the Kaplan book, but I am pretty sure the i is included in the onli9ne material the lesson books and the flash cards.
     
  9. DRHOYA

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    Thanks. I guess you should just always include it, because if the factor is "1" then it is not going to effect the overall equation. And like OsimsDDS said, whether the question states "non-dissociating" or "dissociating" in it.
     
  10. doc toothache

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    Kaplan may be only be talking about non electrolytes and failed to talk about electrolytes.
     
  11. Molar Man

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    yeah "i" is quite important, I also noticed kaplan didnt put that in, but the concept is easy enough to remember. ask yourself before plugging in/solving "Does the solute disassociate? if yes, How much?"
    three examples:
    glucose -> does not so its i is 1
    KNO3 -> does into K+ and NO3- so i=2
    Br(OH)2 -> yes into Br2 and 2(OH) so 1=3
     
  12. harrygt

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    I would also like to point out something. Whenever the questions say that the particles make a dimer in solution [it means two particles join each other], you have to use i = 1/2.
     
  13. DRHOYA

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    Nice tip!
     

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