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Explaining Mass Loss in Differential Distillation

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gpmattos
#1
Jun20-13, 09:44 AM
P: 5
Hi, i just perfomed a differential distillation experiment, the initial solution cointaned ethanol-water solution with 4.9% in mass it weighted 399.63g

Water Mass:381.25

Ethanol Mass:19.96

We distilliled the solution for 20 minutes and obtained two solutions, the distilled had 35.87g and the residue 361.21.We used a refractometer to measure the ethanol mass fraction, we made a calibration curve. And after comparing the mass fraction predicted by that curve we observed a 18% positive error in ethanol concentration. Well no problem so far.

Distilled.
Ethanol Mass: 10.517

Water Mass: 25.353


Residue.
Ethanol Mass: 6.133

Water Mass: 354.987


The point of making this thread is: It was expected that we would lose mass during the experiment, 2.64g. The water loss was negative (-0.5) probably due to the calibration curve error. Now, i'm trying to explain two things mainly: Why and how did we lose mass, and why did ethanol lost more mass?

I don't know if i made any mistakes but i found out that the Schmidt Number for Ethanol is actually lower than Water because Ethanol's difusivity is half that of water.




I don't have a picture of the experimental apparatus, so i'll try to explain since i didn't find nothing similar. It was a standard differential distillation apparatus but the condenser was vertical and quite tall, that's my main concern, being so tall it seems hard to me that so much mass was lost due to evaporation. Well,
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mfb
#2
Jun20-13, 10:48 AM
Mentor
P: 11,904
It was a standard differential distillation apparatus but the condenser was vertical and quite tall, that's my main concern, being so tall it seems hard to me that so much mass was lost due to evaporation.
Does that mean it was open at the top? In that case, evaporation can explain the mass loss, and ethanol evaporates quicker than water. Apart from that: calibration/measurement errors are comparable to the mass loss, so it is hard to see the individual components in the mass loss. It would have been interesting to mix the two liquids again after the experiment, to cancel calibration errors.
gpmattos
#3
Jun20-13, 10:52 AM
P: 5
Yes, it was open at the top, but the column was very tall, thin and the water cooling the condenser was at a considerable low temperature: 10-11 oC. I'm not sure whether i have to use ethanol volatility or Schmidt Number to express why ethanol evaporated more.


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