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Freezing Point

Discussion in 'MCAT Study Question Q&A' started by Revilla, Dec 20, 2008.

  1. Revilla

    Revilla New Member
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    The equation is:

    Tf = Kf*i*m

    If I understand it correctly, the higher the Tf value, the lower the freezing point, meaning compounds with the lowest molecular weight (assuming the amount of solvent and the value of "i" is the same) have the lowest freezing points because their molality will be the highest? Or do I have it backwards?

    If that's true, then for boiling point elevation (Tb = Kb*i*m), the lower the molecular weight, the higher the boiling point (once again, assuming the amount of solvent and "i" values are equal). Where I'm confused is that I thought the first thing you look for when distinguishing boiling point was IM forces and after that, molecular weight. Compounds with the highest molecular weights have the highest boiling points, or so I thought. But given this equation, it doesn't work out.

    Can anyone explain it to me please?
     
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  3. ksmi117

    ksmi117 GEAUX TIGERS!!!
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    Freezing point depression and boiling point elevation only apply to solutions as does the concept of molality. I think where you are confused is that molality = moles of solute/kg of solvent. The molecular weight of the solute does not affect the kg of solvent. Boiling point elevation and freezing point depression are colligative properties (meaning that the substance itself does not matter only the number of particles). One mole of NaCl would have the same effect as one mole of KCl (assuming that you use the same amount of the same solvent).

    Also, the fact that high molecular weights implies high boiling points is true, but not in solutions, which are where the freezing point depression and boiling point elevation equations are used.

    Hope this helps. If not, PM me and I'll try to explain better. Happy studying!
     
  4. Revilla

    Revilla New Member
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    Thanks! You did clear up for me why the molecular weight doesn't when talking about boiling point of solutions, but wouldn't molecular weight affect molality too since you need to find the moles to divide by the kg of solvent?

    So if you're not given moles of NaCl, but are told it's 100g vs. 100g of KCl, you need to find the moles of both and in that case, the one with the lowest mass will have the highest number of moles and if it's equal kgs of solvent, then the one with the lowest mass (i.e. most moles) will have the largest molality and assuming everything else is equal, the bigger Tf. Do I have that backwards? If not, does that mean the one with the biggest Tf will have the lowest freezing point?
     
  5. ksmi117

    ksmi117 GEAUX TIGERS!!!
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    In that case, yes, the NaCl would have the highest boiling point/lowest freezing point, solely because there are more particles of it in solution. When comparing the two solutions in this case, you can't really think of 100 g of each meaning equal amounts, because moles are what matter here not grams.
     
  6. Revilla

    Revilla New Member
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    Okay, thanks! It's starting to make more sense now.
     

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