Freezing Point Depression

**e-diot** · May29-05, 12:18 PM

if i have some of my unknown sample which remains unmelted and undissolved on the walls of the test tube, will this error cause the calculated molecular mass to be too high or too low?

i've thought about this and my guess is that the molecular mass would be too high because the colligative property depends on the concentration of the dissolved particles in the solution right. so less dissolved particles means that the freezing point will be higher. is that right? i'm so stressed :S

also when determining the molecular mass of carboxylic acid using this freezing point depression method the value found is approximately twice the theoretical value. i need to explain why this occurs

i was wondering, is it something to do with how well carboxylic acid dissolves in the solvent? and because (from my text book) it says something about ionic compounds of oxoanions with high charges tend to be insoluble, carboxylic acid would then hardly dissolve in the solution, therefore freezing point temperature would be high causing the "determined" molecular mass to be twice the theoretical?

hrmm i hope i haven't confused anyone. please help me understand if you can

**GCT** · May29-05, 01:49 PM

less dissolved particles will result in a lower freezing point depression.

It is extremely inaccurate to determine the molecular mass of a weak acid by freezing point depression. Why don't you try and calculate what the freezing point depression would have been with the substitution of the actual molar mass of the acid. Your value will be very, very, very low for the freezing point depression. It is most often the case that people will tend to overestimate the freezing point depression from the graphs data provided by such software as measurenet. Your experimental value is quite typical, it's a lesson you'll need to learn.

**Gokul43201** · May29-05, 04:03 PM

Originally Posted by e-diot

if i have some of my unknown sample which remains unmelted and undissolved on the walls of the test tube, will this error cause the calculated molecular mass to be too high or too low?

If you do the calculation, it will tell you the answer.

also when determining the molecular mass of carboxylic acid using this freezing point depression method the value found is approximately twice the theoretical value. i need to explain why this occurs

Might it just be possible that the carboxylic acid itself behaves like it is made up of units where two molecules act as one ? There is a common name for this phenomenon; what is it ?

**Borek** · May29-05, 06:31 PM

Most carboxylic acids are weak, so there should be not doubling in freezing point depression. Not to mention the fact that it is not stated that it is water freezing point depression...

Edit: is it molecular mass doubled, or freezing point depression doubled?

For me that's a clinical example of stupid question asked in the place where there are lots of good questions to ask.

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Borek
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**e-diot** · May30-05, 02:55 AM

Originally Posted by Gokul43201

Might it just be possible that the carboxylic acid itself behaves like it is made up of units where two molecules act as one ? There is a common name for this phenomenon; what is it ?

this phenomenon is called super cooling?

i not sure hehehe

P.S Borek, it is the "determined" molecular mass of the carboxylic acid that has doubled

**e-diot** · May30-05, 03:24 AM

Originally Posted by GCT

less dissolved particles will result in a lower freezing point depression.

hrmm i think i'm getting bit confused with the terms lower freezing point depression and higher freezing point depression.
is lower freezing point depression a lower freezing point temperature? say freezing point of pure substance is 50 and so would a lower freezing point depression be something like 45?
and a higher freezing point depression be something like 48?

**Borek** · May30-05, 03:39 AM

Forget about "lower", it is freezing point depression. Period.

It means that the liquid substance of known freezing point freezes at lower temperature, when there is something dissolved.

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Borek
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**e-diot** · May30-05, 04:07 AM

Originally Posted by Borek

Forget about "lower", it is freezing point depression. Period.

It means that the liquid substance of known freezing point freezes at lower temperature, when there is something dissolved.

ah okay i think i get it now. so then the less dissolved, the freezing point will only drop a little. so because some of the unknown remains undissolved/unmelted on the walls of the test tube then my calculated molar mass would be too high?

**Borek** · May30-05, 10:31 AM

Exactly. If not all is solved you are underestimating number of moles.

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**GCT** · May30-05, 12:51 PM

his phenomenon is called super cooling? i not sure hehehe

no, although you'll probably witness supercooling when performing this experiment, it relates more to van't hoff factor, i.

hrmm i think i'm getting bit confused with the terms lower freezing point depression and higher freezing point depression.
is lower freezing point depression a lower freezing point temperature? say freezing point of pure substance is 50 and so would a lower freezing point depression be something like 45?
and a higher freezing point depression be something like 48?

lower freezing point depression relates to the freezing point of the mixed solution v.s. the pure solvent (in this instance, water). If more solutes are dissolved you'll witness a lower change in freezing point, freezing point of solution will be relatively lower than the pure solvent (colligative property).

again, your deviancy from the actual value is typical. The method is one of the most imprecise (as well as inaccurate) ways of calculating the molar mass. If you would have performed the calculation as I had asked in the my first post, you would have found that the theoretical freezing point depression is almost insignificant, very, very, very small. You would not have been able to witness this with the naked eye by simply observing a graph. Thus your overestimation is typical, it is actually a very good estimation in my opinion.

**Borek** · May30-05, 06:22 PM

Look for dimerization of carboxylic acids. That's what Gokul was referring to in his post.

Best,
Borek
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