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Homework Help: Simple Distillation

  1. Nov 5, 2008 #1
    1. The problem statement, all variables and given/known data
    So I did a simple distillation lab and discovered that my unknown was methanol (boiling point was around 65). But, I just wanted to check something: since the alcohol was impure, shouldn't the boiling point range of the unknown be above pure methanol's boiling point and below that of the impurity? However, my experimental range was between 63 - 64 degrees celsius. So, I was just wondering if that was an error or there's is another explanation (it can't be because the impurity's boiling point is lower than methanol's can it?). I have to give a big explanation/analysis based on my data...but I just wanted to make sure my data is at least correct before I start analyzing in the wrong direction, since I'm not really sure what accounted for the low bp range of my impure methanol..

    **edit: the temperature vs. volume graph of the distillate ressembles that of an ideal solution with two miscible volatile liquids...but Im pretty sure my solution was just ethanol + impurity (about 80% recovery). Would that mean that the distillate..contained both of the two miscible liquids? or just methanol?

    Second question: after I did the experiment, I graphed the temperature vs. the volume of distillate. Then, I was asked to explain the composition of the liquid. Is this possible, finding the % of methanol and % impurity in the unknown? I tried looking online...but didn't really find a way to do it. And if this isn't possible...generally, what do they mean by "what can you conclude about the composition of the unknown liquid"? Do they want you to write how, relatively, volatile the liquid is compared to pure methanol or..?

    Third question (last one i promise): the following data is given:
    pentanol boiling point = 138, water boiling point = 100 (degree C)
    azeotrope boiling point = 95.4, azeotrope composition = 55% water, 45% pentanol.

    Describe the behaviour of a mixture containing 70% water plus 30% pentanol during its simple distillation.

    2. Relevant equations

    3. The attempt at a solution

    Ok, so for the last question, I graphed the data on a bp-composition diagram. And from that...I predict that the mixture of 70% water and 30% pentanol should start boiling at 92 ish degrees...a temperature between the azeotrope bp and water bp. As well, the liquid being boiled should be water (or is it both liquids? I'm unsure about this part). Um...also, the liquid in vapour pressure at that point should be more rich with water than pentanol (but there should be >30% of pentanol in vapour pressure at that point). The distillate should contain water, separated from the pentanol.

    Uh...and I'm not really sure what other behaviour would be observed. Any ideas (or has what I said so far been correct at least)?
    Last edited: Nov 5, 2008
  2. jcsd
  3. Nov 6, 2008 #2


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    Have you read about azeotropes? Also you mention methanol , ethanol , and pentanol - sometimes it seems that you mix them up.

    Please specify the experimental objective - were you simply finding boiling points of pure liquids or was this a lesson in ideality?

    Please specify the experiment question in its exact form - one by one.
  4. Nov 6, 2008 #3
    ah each of the questions are separate. for the first question concerning methanol, that one doesn't really have a question. It just says "Discuss and conclude", so I just wanted to check my data and stuff. The second question is an extension of the first, still with methanol, and the question is: based on the temperature vs. distillate's volume graph obtained from the experiment, what can you conclude about the composition of the unknown solution?

    Third question is separate. It has to do with pentanol and water, as an azeotrope. And the question is posted there (Describe the behaviour of a mixture containing 70% water plus 30% pentanol during its simple distillation.). Sorry bout the confusion though.

    **ethanol thing was purely a typo.

    edit: the purpose of the experiment is basically to identify an unknown solution and I guess, explain its composition in terms of trends you get from graphing the solution's boiling temperature vs. the volume of distillate that you get.
  5. Nov 6, 2008 #4


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    Here is a good link for the moment. Scroll towards the latter sections.


    Should you know the volume of the first distillate and its boiling temperature you may be able to ascertain the identity as well as the composition of the unknown solute as well as the solvent - assuming that you found the its boiling point. As for your other questions they seem general , you need to introduce the problem by stating it and then pose a solution.
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