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Homework Help: Maximum Percent Recovery

  1. Oct 8, 2013 #1
    1. The problem statement, all variables and given/known data

    One often stated advantage of ethanol/water as a recrystallization solvent is that the mixture behaves like ethanol at reflux and like water at room temperature or below. Assuming that 24 mL of the solvent is used per 1 g of crude product and recrystallization and recovery of the product is done at room temperature, what is the maximum percent recovery that can be achieved?

    2. Relevant equations


    3. The attempt at a solution

    I thought perhaps we could convert 24 mL of solvent into 24 g of solvent using water's density as a conversion factor (1.00 g/mL) since the solvent acts like water during recrystallization and recovery in this question. Then I took the ratio of crude product to solvent:

    1g product / 24g solvent * 100% = 4.2% lost

    Therefore 100% - 4.2% = 95.8% recovery.

    Not only do I not have confidence that this is correct, but I also don't know why this calculation makes sense, if at all.

    Thanks a lot for any help!
  2. jcsd
  3. Oct 8, 2013 #2


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    Staff: Mentor

    I don't see how to solve the problem at all, without information about what the product solubilities are.
  4. Oct 8, 2013 #3
    There must be a way.. This question didn't even provide so much as the identity of the product. Also, this question seems to have been used for at least 7 years because a Google search of it shows results from 2006 :P (couldn't find any answers, however), so it must be answerable for it to not have been removed.
  5. Oct 9, 2013 #4


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    Staff: Mentor

    This is a faulty logic - conclusion doesn't follow from the premises :tongue:

    But if there is an answer, I would love to see it. If you will find anything, please share.
  6. Oct 9, 2013 #5


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    Homework Helper
    Gold Member

    Yes, you can't solve this quantitatively with information given. Maybe they are only asking for a qualitative answer. When you mix with the ethanol/water mixture, some of the stuff will dissolve and some won't. Then when you heat it should all dissolve (because you've fixed quantitities so that that happens). When you then cool the maximum that will crystallise will be what didn't dissolve when cold in the first place.

    Or else sometimes it doesn't all dissolve and you filter it hot. The purer product is in the filtrate and you hopefully recrystallise it. Sometimes the filter funnel is heated by a jacket to stop it cooling too much and recrystallising too soon before separation in the funnel. Sometimes some of the ethanol is distilled off. Sometimes it is driven off by heating over a flame in a beaker and then a furious chemistry teacher comes shouting You're more of a fool than I thought you were! :biggrin:

    It's a long time since I did any of this, especially the last bit, but I think this is right and is all you can say with the info given.
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