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Topic: 4 Questions from Colligative Properties of solution experiment  (Read 10081 times)

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Offline noppawit

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I would like to ask 4 questions from Colligative Properties of solution experiment.
In this experiment, I used Naphthalene as solvent. Its Kf = 7.45 C kg mol-1

1) A phenomenon called supercooling is frequently encountered in this experiment. In supercooling, a solution momentarily drops below its freezing point, and then warms up again, before solidification begins. What even is likely to give rise to supercooling?

2) The molal freezing point constant Kf is a property of the solvent, not the solute. What does this say about the fact that freezing point depends on the amount of solute, rather than on the solute’s nature?

3) Look up the freezing point constant, Kf for naphthalene in a handbook. How closely does your value for Kf compare? What might have led to your obtaining a different value?

4) A phenomenon that happens sometimes during freezing point depression experiments is that the solute is affected in some manner by the solvent. One common occurrence is for a solute to dimerize; that is two solvent molecules combine to produce a single double molecule (a dimer). What effect would there be on a molecular weight determination if the solute were to dimerize?

Thank you  ::) :P

Offline nj_bartel

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Re: 4 Questions from Colligative Properties of solution experiment
« Reply #1 on: August 11, 2008, 01:06:28 AM »
You need to show attempts at these questions.

Offline noppawit

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Re: 4 Questions from Colligative Properties of solution experiment
« Reply #2 on: August 11, 2008, 01:28:52 AM »
Ok. For what I have tried,

Question No. 1) I know that supercooling means the process of chilling a liquid below its freezing point, without it becoming solid. But I don't understand the question that What even is likely to give rise to supercooling?

Question No. 2) I only know that unit of Kf is degree C kg/mol. Its unit makes me know that Kf must be involved with mass or amount of solute. But how can I describe it, that I don't know.

Question No. 3) I don't understand the question. I know that Kf in my experiment is 7.45 C kg mol-1. I don't know that which Kf I should compare with, I have only Kf of naphthalene that is given.

Question No. 4) This question I cannot give any attempt, because I absolutely don't know about this.

Offline nj_bartel

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Re: 4 Questions from Colligative Properties of solution experiment
« Reply #3 on: August 11, 2008, 01:36:42 AM »
Ok. For what I have tried,

Question No. 1) I know that supercooling means the process of chilling a liquid below its freezing point, without it becoming solid. But I don't understand the question that What even is likely to give rise to supercooling?


What happens when a liquid is frozen?  What forms?

Offline nj_bartel

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Re: 4 Questions from Colligative Properties of solution experiment
« Reply #4 on: August 11, 2008, 01:39:56 AM »

Question No. 3) I don't understand the question. I know that Kf in my experiment is 7.45 C kg mol-1. I don't know that which Kf I should compare with, I have only Kf of naphthalene that is given.

So you're experimental Kf and given Kf were the same?  If that's the case, then you both controlled you experiment very well and got lucky  ;D  However, your lab TA would probably like to see more than that.  So, if the experimental Kf would have come out lower than the given Kf, what could have caused that?  If the experimental Kf was higher?  What affects Kf?

Offline nj_bartel

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Re: 4 Questions from Colligative Properties of solution experiment
« Reply #5 on: August 11, 2008, 01:46:29 AM »

Question No. 4) This question I cannot give any attempt, because I absolutely don't know about this.

I'm assuming it's just talking about the MW of the solute, by itself.  So take into account, my explanation's based on that assumption.  Think of it on a large scale.  You have a mole of ice cubes of MW 5 grams.  Those ice cubes dimerize - each single ice cube finds another single ice cube to form a large ice cube.  How much does each one of these large ice cubes weigh in relation to the small (undimerized) ice cubes?  Now you have the same mass of ice, but it's all dimerized into large ice cubes, 5 grams.  How many moles of ice cubes do you now have though?  What does this mean happens to the MW?

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