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Freezing point depression

Tokspor

Full Member
Jul 3, 2009
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  1. Pre-Medical
    This is from Kaplan's Qbank...

    What is the freezing point of a saturated aqueous solution of 0.1 m lithium fluoride (Kf = -1.86 K/m)?

    The answer is supposedly -0.186 degrees Celsius because delta T = (-1.86 K/m)*(0.1 molal).

    But what about the van't Hoff factor? Shouldn't everything be multiplied by 2 to account for the dissociation of lithium fluoride into two ions?
     

    G1SG2

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    5+ Year Member
    May 2, 2008
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    1. Pre-Medical
      This is from Kaplan's Qbank...

      What is the freezing point of a saturated aqueous solution of 0.1 m lithium fluoride (Kf = -1.86 K/m)?

      The answer is supposedly -0.186 degrees Celsius because delta T = (-1.86 K/m)*(0.1 molal).

      But what about the van't Hoff factor? Shouldn't everything be multiplied by 2 to account for the dissociation of lithium fluoride into two ions?

      Yeah, I don't know why they didn't include it. Maybe because the real van't Hoff factor is so small (given the electrostatic attraction present in LiF), that they ignored it?
       
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      Jepstein30

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      Jun 29, 2011
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        it's a poorly written question.. it should specifically tell you to use an estimated van't hoff factor (2) or provide the actual one (somewhere between 1-2)

        it's possible the van't hoff factor isn't actually 2 because it doesn't fully dissociate.. but as posed, there's no reasoning to make that assumption.

        was there an answer choice that fit using 2 as the van't hoff factor? because if not, while its still poorly written.. it's still a fair question. MCAT will be more straightforward with this kind of thing though.
         

        brood910

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        Jul 22, 2011
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          it's a poorly written question.. it should specifically tell you to use an estimated van't hoff factor (2) or provide the actual one (somewhere between 1-2)

          it's possible the van't hoff factor isn't actually 2 because it doesn't fully dissociate.. but as posed, there's no reasoning to make that assumption.

          was there an answer choice that fit using 2 as the van't hoff factor? because if not, while its still poorly written.. it's still a fair question. MCAT will be more straightforward with this kind of thing though.

          Kaplan still did a bad job here as salts are usually assumed to have i = 2 when it is not mentioned.
           

          Jepstein30

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          Jun 29, 2011
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            Kaplan still did a bad job here as salts are usually assumed to have i = 2 when it is not mentioned.

            Yea? Could be.. don't remember enough questions on the topic.

            I'd think AAMC will clarify what van't hoff factor they'd want you to use though.

            I mean.. technically.. Kaplan IS right.

            Based on solubility rules, flourides are generally insoluble (and if you look it up, LiF is not soluble in water). So you would use a van't hoff factor of 1.

            It's just, I don't think AAMC really expects you to memorize the solubility rules so that information should be provided in some form.
             

            brood910

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            Jul 22, 2011
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              Yea? Could be.. don't remember enough questions on the topic.

              I'd think AAMC will clarify what van't hoff factor they'd want you to use though.

              I mean.. technically.. Kaplan IS right.

              Based on solubility rules, flourides are generally insoluble (and if you look it up, LiF is not soluble in water). So you would use a van't hoff factor of 1.

              It's just, I don't think AAMC really expects you to memorize the solubility rules so that information should be provided in some form.

              OH RIGHT. Nice catch there.
              Yeah, it is insoluble, so regardless of whether it is a salt or not, we should assume i = 1.

              Many questions in TBR do not give you any values of i, and they expect you to know these values according to solubility rules though.
               
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