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Homework Help: Quick silly dilution question

  1. Sep 17, 2014 #1
    1. The problem statement, all variables and given/known data
    The mols of ethanol is 1.72x10^-4. I am trying to find its volume. However, I am confused on what the molarity of this solution is. (Molarity here is in mol/L)

    (the 10mL sample contained 1.72x10^-4 mols of ethanol, but it was diluted in 90 mL water)

    The lab said:
    "Dilute the 10.0 mL ethanol sample as such: (10.0 mL ethanol in 90.0 mL of distilled water, for a total of 100.0 mL)."

    3. The attempt at a solution

    The mols of ethanol is 1.72x10^-4. Do I divide this figure by .01 liters? Or do I do it by .1? I think it's one or the other but either way I need this calculation perfect or the rest of the lab is moot!
    Last edited: Sep 17, 2014
  2. jcsd
  3. Sep 17, 2014 #2


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    Staff: Mentor

    If you want to find the molarity, you divide the number of moles by the volume (here: .1 liter).

    I don't see which volume you want to find.

    10ml ethanol plus 90ml distilled water won't give exactly 100ml, but that's probably a negligible detail.
  4. Sep 17, 2014 #3
    Oops, sorry! My original post was a bit unclear.

    The original dilution scheme was 10 mL of the ethanol in 90mL water [I think my teacher isn't really concerned with the negligible detail(s)]

    1.72x10^-4 mols of ethanol is the mols of ethanol oxidized by dichromate ion in my lab. I am trying to find the volume of those mols. The procedure included preparing a solution by adding 10 mL ethanol into 90 mL of water. We shook it up and added into a seperate flask 1 mL of that solution and 21 mL of 0.01M dichromate in 5M H2SO4.

    Here's the reaction if it is relevant:
    2 Cr2O72− + 16 H+ + 3 C2H5OH → 4 Cr3+ + 11 H2O + 3 CH3COOH

    Hopefully that clears things up! For such a simple dilution question I can't believe I'm having trouble with it, of all things to be troubled with in this lab.
  5. Sep 17, 2014 #4
    how silly. I think I just had to convert the grams to mL using ethanol's density - (0.789g/mL).

    Resulting figure would be of the 10 mL ethanol originally put in, 0.01 mL was oxidized.

    7.92x10^-3 g ethanol X (1 mL/0.789g) = 0.0100mL

    (7.92x10^-3 g of ethanol = 1.72x10^-4 mols of ethanol )
  6. Sep 18, 2014 #5


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    Staff: Mentor

    You took 1 mL out of 100 mL of the diluted solution, so 1/100th of the original sample. If 0.01 mL is 1/100th of 10 mL, then you got it right.
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