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stephaniem3562
Joined: 02 Mar 2010
Posts: 2
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 Posted: Thu Mar 18, 2010 1:16 pm Post subject: |
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Acetic acid and sucrose are non volatile solutes that are added to water. Hence you add up their molalities. What I dnt understand is why not take the "i" into consideration? i for acetic acid is 2 and for glucose, i=1. Therefore total i= 3. Thus change in temp= Kf* (0.5m+0.5m)*3.
Why is this not correct? |
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mcat_premed3832
Joined: 19 Oct 2006
Posts: 428
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| Posted: Mon Mar 22, 2010 10:42 pm Post subject: |
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This problem aims for a concept often tested on the MCAT (though it's name is rarely given on the test!): Colligative properties.
These are properties that depend on the number NOT the type of particles.
| Quote: | | i dont quite understand why you add 0.5 m acetic acid +0.5m sucrose? |
Boiling/freezing point elevation/depression is a colligative property and so the total number of particles must be included in the calculation.
| Quote: | | Acetic acid and sucrose are non volatile solutes that are added to water. Hence you add up their molalities. What I dnt understand is why not take the "i" into consideration? i for acetic acid is 2 and for glucose, i=1. Therefore total i= 3. |
It's good that you considered that line of thinking because it means you are clear on the "colligative properties" idea. However, acetic acid is a very weak acid. You may remember doing problems showing acetic acid having a Ka around 10^-5 (so that's 1 in 10 000!!!). |
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tnfifn3735
Joined: 22 Jan 2010
Posts: 13
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| Posted: Thu Jun 10, 2010 1:28 pm Post subject: |
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| mcat_premed3832 wrote: |
| Quote: | | Acetic acid and sucrose are non volatile solutes that are added to water. Hence you add up their molalities. What I dnt understand is why not take the "i" into consideration? i for acetic acid is 2 and for glucose, i=1. Therefore total i= 3. |
It's good that you considered that line of thinking because it means you are clear on the "colligative properties" idea. However, acetic acid is a very weak acid. You may remember doing problems showing acetic acid having a Ka around 10^-5 (so that's 1 in 10 000!!!). |
Okay, even if acetic acid is a weak acid, one would still be left with i=1 and from the glucose i =2. Which means the i=2. Therefore the temperature should have decreased by 4C as oppose to 2C. I know that 4 was not a choice. But why isn't 4 the correct answer. |
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jellywing_2058
Joined: 04 May 2009
Posts: 177
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| Posted: Fri Jun 18, 2010 2:35 pm Post subject: |
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No the answer given is correct.
Sucrose as per the passage (not glucose) is not an electrolyte and is a sugar (organic molecule) that does not ionize and so the Van Hoff factor (i) for sucrose (or even glucose) will be = 1 (not 2).
So, for sucrose i = 1 and the answer is in fact D as given and the explanation is fine. |
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tnfifn3735
Joined: 22 Jan 2010
Posts: 13
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| Posted: Tue Jun 22, 2010 11:58 am Post subject: |
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| jellywing_2058 wrote: | No the answer given is correct.
Sucrose as per the passage (not glucose) is not an electrolyte and is a sugar (organic molecule) that does not ionize and so the Van Hoff factor (i) for sucrose (or even glucose) will be = 1 (not 2).
So, for sucrose i = 1 and the answer is in fact D as given and the explanation is fine. |
That makes sense.
PS. Thanks for all the replies. Your posts are quite helpful. |
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