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Finding volume content

  1. Oct 9, 2011 #1
    1. The problem statement, all variables and given/known data

    The question is as following:

    2. Relevant equations

    3. The attempt at a solution

    I have absolutely no idea on how to solve this problem. Usually problems indicate some quantity which we can substitute into an equation.
    Any tips will be greatly appreciated.
  2. jcsd
  3. Oct 9, 2011 #2


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    You know that the amount of oleic acid in the final mixture is 1 part in 400. So all you have to do is figure out the volume of a single droplet, and the volume of oleic acid in that droplet will be 1/400 of that droplet volume.

    So, what is the volume of a droplet? Well, 97 droplets have a volume of 1 cm3, so 1 droplet has a volume of ____?
  4. Oct 9, 2011 #3
    Hi, thanks for your reply.
    Hm will it be 1/97 cm[itex]^{3}[/itex] , if so then would I multiply this fraction by 400 ?
  5. Oct 9, 2011 #4


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    That's right.
  6. Oct 9, 2011 #5
    Thank you very much! The wording of the question brought about confusion to my mind.This question has another parts as well, which I'll try to solve.
  7. Oct 9, 2011 #6


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    Are you sure multiplication by 400 equals the correct volume of oleic acid?
  8. Oct 9, 2011 #7
    Volume of oleic acid in one droplet ?
    why is it wrong ?
  9. Oct 9, 2011 #8


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    Because the oleic acid was being diluted in methanol.
  10. Oct 9, 2011 #9


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    Yeah, sorry, I missed that. I thought he meant multiply the denominator by 400 (i.e. multiply by 1/400 just the way I told him to).
  11. Oct 10, 2011 #10
    Oh I see.. I initially thought of multiplying by 1/400 , i even had that posted as an answer until I changed my mind.Some misunderstanding from both ends. Now it's clear. Thanks.
  12. Oct 10, 2011 #11
    So here's my final answer for this question: 1/97 x 1/400 ~ 2.6x10^-5 cm3

    The questions which follow on are:

    The surface of a tank of water is dusted with fine powder. A single drop of the
    diluted mixture is allowed to fall on the water surface. The alcohol component mixes
    with the water and the oil forms a monolayer film on the water surface, displacing
    the dust so it is clearly visible, with a diameter of 9.4 cm. What is the thickness of
    the film?

    I have answered this: All that was needed to be done was using the Volume = Area x thickness.My final answer is: 3.7x10^-7 cm.

    Next question's:
    The shape of an oleic acid molecule can be approximated by a cuboid l ´ l ´ 7l .
    One end (COOH) loves water (hydrophilic) and the other end hates it (hydrophobic).
    Assume that these molecules stick out of the water and form a monolayer close
    packed in a square arrangement. What are the dimensions of the molecule?

    What does the statement which I have highlighted in bold mean ? do I find out the unit dimension of the molecule? I know lxlx7l denotes volume of the molecule.

    Again thanks for your help.
    Last edited: Oct 10, 2011
  13. Oct 10, 2011 #12
    Never mind I have found the answer for this. Thanks everyone for your help!
  14. Oct 10, 2011 #13
    I need a quick tip on how to solve one of the last bit of this question. The question basically wants me to derive Avogadro's number of oleic acid. So the molecular mass I have got is : 282.
    Now I know moles = Mass/ Mr.
    To get mass I will have to use density = Mass/ volume.
    But once I sub in the numbers.. I get something in the range of 5x10^24! :(

    My volume for oleic acid in one droplet was: 2.6x10^-5 (cm3 , do i divide this by 100 to get m3, or is it 1x10^-6 ? )
    Density in the question is given to be 890 kg/m3.

    Any help will be much appreciated. Thanks!
  15. Jan 29, 2012 #14
    How do I find out the volume of something that is 36w - 16h - 16d. Would it just be 36 * 16 * 16.
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