Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: FthkuMan on September 10, 2016, 09:42:35 AM
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Hi guys, I've got the following question that I could use some help with:
You have two solutions, A and B.
Solution A contains a mixture of water and Glycerol (V\V % is 1:1)
Solution B contains a mixture of Ethanol and water (V\V % is 1:1)
Data: Glycerin - Boiling point at 290°C, freezing point at 18.6°C, Molar mass 92 g/mol
Ethanol - Boiling point at 78°C, freezing point at -114°C, Molar mass 46 g/mol
Water - Boiling point at 100°C, freezing point at 0°C, Molar mass 18 g/mol
Determine whether the following is true or false:
1. The B.P of B is higher than that of water.
2. The B.P of A is higher than that of water.
3. The F.P of B is lower than that of water.
4. The F.P of A is lower than that of water.
To me it would seem all but 1 is true. 4 is problematic as I know glycerin is used as an antifreeze, yet it doesn't seem to make sense. Help?
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Neither BP nor FP changes linearly with the mixture composition.
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Why do you think 4 is problematic?
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Why do you think 4 is problematic?
Because its freezing point is above that of water's, so how is it used as an antifreeze?
And Borek, forgive me but I'm not sure I understand.
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Plot describing the freezing point of the mixture as a function of its composition is not a straight line (it is only for so called ideal solutions).
The same can be said about boiling points, google azeotrope.
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I understand. But in the question I was told that the volume ratio is 1:1, so what does that tell me? would that mean the concentration is 0.5M for the glycerin, and so according to your graph would indeed lower the F.P?
Sorry for all the questions, I just want to make sure I understand this fully.
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From your molar ratio you can easily estimate (exact calculation are not neccesarily) mass percent of glycerin as between xx and yy % and find freezing point of such mixture on the picture.
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Plot describing the freezing point of the mixture as a function of its composition is not a straight line (it is only for so called ideal solutions).
The same can be said about boiling points, google azeotrope.
at the graph... :o
Why do you think 4 is problematic?
Because its freezing point is above that of water's, so how is it used as an antifreeze?
And Borek, forgive me but I'm not sure I understand.
Sorry, I typed that without looking at the given data. I just made a natural assumption that glycerin and water mixture would have a lower FP.
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As a sidenote, the "freezing point" should be replaced by the melting point as much as possible, among others because its measure is better repeatable. And fp can be understood as "flash point", so mp is better.