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Homework Help: Polystyrene repeat unit molecular weight.

  1. Sep 27, 2017 #1
    Edit: thread moved from non HW forum.



    I'm trying to calculate the unit molecular weight of polystyrene. I'm given the structure, and I have to count all the elements in the structure. To calculate the weight, I keep counting 8C & 7H. The correct answer is 8C +8H. I'm having a hard time deterring where the additional H is coming from.
     
    Last edited by a moderator: Sep 27, 2017
  2. jcsd
  3. Sep 27, 2017 #2

    jim mcnamara

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    Staff: Mentor

  4. Sep 27, 2017 #3

    Borek

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    Can you draw the unit and post it as a picture?
     
  5. Sep 27, 2017 #4
    Picture from the text book.
     

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  6. Sep 27, 2017 #5
    Posted a picture from the text book. Sorry about the wrong thread posting.
     
  7. Sep 27, 2017 #6

    Borek

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    How many hydrogen atoms on the ring? How many hydrogen atoms on the side chain?
     
  8. Sep 27, 2017 #7
    I see 4 in the ring, and 3 in the structure. Idk where I'm leaving the other H atom off.
     
  9. Sep 27, 2017 #8

    epenguin

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    There are 5 in the ring, one on each C except the C joined to the rest of unit..
     
  10. Sep 28, 2017 #9

    Borek

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    And - as epenguin said - that's where you are wrong. You were probably mislead by the picture and the way it depicts the hydrogen atoms on the alkane chain.

    Imagine just benzene - how many hydrogens on the ring? How many hydrogens will be left on a mono substituted ring?
     
  11. Oct 24, 2017 #10
    The benzene ring has 6 carbons and 6 hydrogens. There side chain takes the place of only of the hydrogens on the ring, leaving only 5 hydrogens on the ring. The side chain has 3 hydrogens. So, add 3 hydrogens from the side chain + 5 hydrogens from the benzene ring = total 8 hydrogens.
     
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