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Need an explanation -- Consider pure water separated from...

  1. Dec 3, 2016 #1
    • Please post this type of questions in HW section using the template.
    Consider pure water separated from an aqueous starch solution by a semipermeable membrane, which allows water to pass freely but not starch. After some time has elapsed, the concentration of starch solution
    a. will have increased
    b. will have decreased
    c. will not have changed
    d. might have increased or decreased, depending on other factor.
    e. will be the same on both sides of the membrane

    The answer key says that the answer is b. will have decreased. This doesn't make sense to me. My understanding is that if the water is gone, the volume the starch is occupying has decreased but the amount of starch stayed the same. Intuitively, this would lead me to believe that the concentration has increased (assuming by concentration they mean something like molarity, where molarity = moles of solute/ volume ). My dad, who has a PhD in Chemistry, agrees with me. Can someone explain to me why it would decrease? Thanks!
     
  2. jcsd
  3. Dec 3, 2016 #2

    Borek

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    Staff: Mentor

    Why do you think water will be gone?
     
  4. Dec 3, 2016 #3
    Well, the question says "consider pure water separated from an aqueous solution by a semipermeable membrane". Isn't the water being separated from the starch solution?
     
  5. Dec 3, 2016 #4

    Borek

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    Staff: Mentor

    What it means is you have two solutions and a semipermeable membrane between them (separating them as in "don't allowing a direct mix"), it doesn't say anything about what is happening to water (other than that it can freely move - but it is up to you to decide which way it moves).
     
  6. Dec 3, 2016 #5
    Oh, okay that makes more sense. Thanks!
     
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