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Topic: Freezing Point Question  (Read 11892 times)

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photogrl

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Freezing Point Question
« on: July 24, 2005, 01:10:39 AM »
Which of the following solutions would have the lowest freezing point?

1M MgCl2

1M NaCl

1M C6H12O6

1M Ca3N2


I believe the correct answer is 1M C6H12O6, because doesn't the freezing point of a solution have to do with the number of molecules of said solution? If so, then the glucose would be the right answer.

Or am I absolutely dead wrong? Help? Please?

Thank you. :)
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Offline Yggdrasil

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Re:Freezing Point Question
« Reply #1 on: July 24, 2005, 01:27:43 AM »
If I remember correctly, freezing point depression is bases on molality, not molarity.  Take a look at the definition of the two, and see if you can come up with an answer based on that.  Feel free to post your guess, and we'll check whether it's right.
« Last Edit: July 24, 2005, 01:27:59 AM by Yggdrasil »
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photogrl

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Re:Freezing Point Question
« Reply #2 on: July 24, 2005, 01:41:01 AM »
OK.....my guess is now 1M MgCl2...

*wrings hands*

Is that right? I'm not completely understanding the freezing point concept.  :(
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Hgrigsby

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Re:Freezing Point Question
« Reply #3 on: July 24, 2005, 02:06:24 AM »
The only way I could solve that is if I had more info, the #kg in the solution (to solve for Molality) not to mention the mols of the solute.  If you have that, then I can happily give you an equation. But that is as much as I can help.
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photogrl

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Re:Freezing Point Question
« Reply #4 on: July 24, 2005, 02:09:26 AM »
Sorry, but the question is written verbatim. I don't have any more info to give you.  :(
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Offline xiankai

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Re:Freezing Point Question
« Reply #5 on: July 24, 2005, 02:33:58 AM »
from my point of view i'll think it in terms of covalent/ionic bonds in general. since covalent bonded substances usually have low freezing points compared to the ionic bonded substances, i'll pick glucose.
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Offline Yggdrasil

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Re:Freezing Point Question
« Reply #6 on: July 24, 2005, 03:06:40 AM »
The answer has nothing to do with the properties of the solute.  Rather, it just depends on the molality of your solition.  Since molality is mass of solute/mass of solvent, it stands to reason that the solute with the highest greatest mass will have the greatest molality (since all the solutions are at 1.0M, meaning they all have equal numbers of solute molecules per unit volume).  Comparing the molecular weights of all the solutes, the sugar (C6H12O6) has the highest molecular weight, and therefore the sugar solution has the highest molality.  Therefore, the sugar solution has the lowest freezing point.

So your original guess (and xiankai's) were correct, but for the wrong reasons ;)
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Offline Borek

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Re:Freezing Point Question
« Reply #7 on: July 24, 2005, 03:38:39 AM »
The answer has something to do with the properties of the solute. Think about effects of dissociation - in which solution molality of ALL dissolved ions/particles will be the highest?

Glucose solution is NOT the correct answer.
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photogrl

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Re:Freezing Point Question
« Reply #8 on: July 24, 2005, 06:34:18 AM »
In as close as you can get to layman's terms....please tell me why the glucose is not the right answer?

Thank you. :)
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Offline Borek

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Re:Freezing Point Question
« Reply #9 on: July 25, 2005, 04:05:21 AM »
Freezing point depression is proportional to number of entities (be it ions or particles) swimming freely in the solution. Think in which solution number of such entities (for the same molal concentration) will be the highest.
« Last Edit: July 25, 2005, 04:09:14 AM by Borek »
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Offline AWK

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Re:Freezing Point Question
« Reply #10 on: July 25, 2005, 04:15:34 AM »
Quote
1M Ca3N2
This part of problem is very odd. Ca3N2 is insoluble in almost all solvents, in water it reacts to form almost insoluble Ca(OH)2 and ammonia.
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Offline xiankai

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Re:Freezing Point Question
« Reply #11 on: July 25, 2005, 08:11:47 AM »
in what context does this question happen to be in?
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Offline Borek

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Re:Freezing Point Question
« Reply #12 on: July 25, 2005, 11:39:59 AM »
Quote from: AWK on July 25, 2005, 04:15:34 AM
This part of problem is very odd. Ca3N2 is insoluble in almost all solvents, in water it reacts to form almost insoluble Ca(OH)2 and ammonia.

Yeah, I didn't touch that as there were (are?) problems with the much more basic stuff.

Seeing such things I always wonder if it is a question that is meant be tricky or is it stupid teacher.

Taking things literally this solution should have the lowest freezing point - 3NH3 plus some dissolved Ca(OH)2 gives more then MgCl2  :)
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Offline Yggdrasil

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Re:Freezing Point Question
« Reply #13 on: July 25, 2005, 01:22:31 PM »
Quote from: Borek on July 24, 2005, 03:38:39 AM
The answer has something to do with the properties of the solute. Think about effects of dissociation - in which solution molality of ALL dissolved ions/particles will be the highest?

Glucose solution is NOT the correct answer.

Ah, reversed the definition of molality in my head.  Thanks for the correction.
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photogrl

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Re:Freezing Point Question
« Reply #14 on: July 26, 2005, 12:47:49 PM »
Thanks for all the help...I found out the answer to that question was the Ca3N2. I had a lot of people tell me it was MgCl3, and even explain WHY it was MGCl2...and I got it wrong anyway. Oh well.
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