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Iodine Numbers

  1. May 11, 2010 #1
    Hi,

    I understand that the iodine number is the number of moles of iodine reacting with one mole of fat/oil which indicates the number of double bonds present in the fat/oil molecule.

    However I am having trouble calculating them.

    Can someone show me how to solve the following question please?

    0.014 moles of a particular oil was found to react exactly with 14.2g of iodine. Calculate the number of moles of iodine that reacted and state what can be deduced about the structure of the oil from this information.

    This is as far as I got:

    Number of moles of iodine: 14.2/126.9 = 0.112mol
     
  2. jcsd
  3. May 11, 2010 #2

    Borek

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    Staff: Mentor

    Is it a homework question?

    What is molar ratio of iodine and hydrocarbon?

    --
    methods
     
  4. May 11, 2010 #3
    Thats all the information that was given, and Its just a revision question - not homework.
     
  5. May 11, 2010 #4

    Borek

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    So use it to calculate molar ratio.
     
  6. May 11, 2010 #5
    I have tried everything I know. With respect, if I knew how to do these questions then I wouldn't be asking. So if I could see how someone else worked it out, I will be able to see how its done, so I can apply it to similar questions.

    Sorry for the trouble, I'm quite desperate.
     
  7. May 11, 2010 #6
    If I had 4 apples and 6 bananas, what is the ratio of bananas to apples?
     
  8. May 11, 2010 #7
    Are you serious?!

    3:2
     
  9. May 11, 2010 #8

    Borek

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    SJB was serious, but you are not serious. You know how to calculate fruit ratio, but not how to calculate molar ratio?

    If you have 0.112 moles of iodine and 0.014 moles of hydrocarbon, what is their molar ratio?
     
  10. May 11, 2010 #9
    8...?

    Is that the number you are talking about?

    Because I had already calculated 8, but the answer in my book says 0.0559 mols....?
     
    Last edited: May 11, 2010
  11. May 11, 2010 #10

    Borek

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    Ah, you made a mistake at the very beginning, I have not spotted it earlier. There is no such thing as atomic iodine.
     
  12. May 11, 2010 #11
    Seems we have two questions here.

    First, how many moles of iodine, for which you have calculated (correctly) as 0.112mol of
    atoms - how does iodine usually exist?

    (looks like Borek has just beaten me to this)

    Then, moving on, how does iodine react with double bonds - what is the ratio there?
     
  13. May 11, 2010 #12
    Oh! Because iodine is diatomic, so there are 2 iodine atoms for every 0.112mols, for 1 iodine atom there are 0.056 mols, so that means the molar ratio is 4! So there are 4 double bonds?!

    The answer 0.056 is approximately equal to 0.0559!

    Thankyou thankyou soo much!

    So you always have to find one atom of iodine?
     
  14. May 11, 2010 #13
    Thank you Mr. Borek. I just could not figure out where I had gone wrong.
     
  15. May 11, 2010 #14

    Borek

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    Huh? Please elaborate.

    --
     
  16. May 11, 2010 #15
    I just meant that the moles of iodine that I had initially calculated (0.112 mol) applied to I2 (a molecule of iodine), however I was asking instead of finding I2, the goal was to find just the number of moles of I.

    Does that make sense?
     
  17. May 12, 2010 #16

    Borek

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    Quite the opposite. What you have initially found was number of moles of I (atomic iodine), while you should have been looking for moles of I2. Two reasons for that - first, iodine is diatomic. Second, it takes iodine molecule (two atoms) to react with double bond.

    --
    methods
     
  18. May 12, 2010 #17
    Thank you, I'm glad that's resolved.
     
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