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Homework Help: Distillation Lab Question!

  1. Oct 28, 2007 #1
    Hey everybody, I just completed a small distillation lab for my high school chemistry class. We took a sucrose/water solution, added yeast to ferment it, and distilled it into ethanol for about an hour in the lab.

    I'm trying to write my lab report, and I've hit a huge stumbling block! One of the questions asks to determine the theoretical yield using the balanced equation we had to come up with, but it also says keep in mind that only 12% of the sucrose will be converted to ethanol Does this 12% factor into anything! Heres my work:

    C12H22O11 -> 4C2H5OH + 4CO2

    We used 49.36 g of sucrose in the original solution FYI

    (49.36 g) * (1 mol sucrose/342.34g) * (4 mol ethanol / 1 mol sucrose) * (46.08 g / 1 mol ethanol) -> 26.99 g ethanol

    Do I do anything with that 12% or is that my theoretical yield right there!?
  2. jcsd
  3. Oct 29, 2007 #2

    jim mcnamara

    User Avatar

    Staff: Mentor

    I'm not sure you read something correctly. Or I don't understand what you said.

    Saccaromyces cerevisae (yeast) can convert sugar (fructose, sucrose) into ethanol.
    When the concentration of ethanol gets to about 12% in the medium, the yeast shuts off
    alcohol production. It sort of "toxifies" itself, so to speak.

    All this presumes that you used brewer's yeast. Other yeasts may not fare as well.

    In other words - alcohol percent depends on the starting concentration of sugar and the maximum possible concentration based on the volume of water left, of ethanol. For example, you start with 20% sucrose solution, then yeast will shut off before using up all of the sucrose.

    You never stated the volume of water used - or I can't find it...

    Is that what you are after....?
    Last edited: Oct 29, 2007
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