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Law of multiple proportions (need method check please)

  1. Oct 19, 2015 #1
    1. The problem statement, all variables and given/known data
    DfXmKL6.jpg

    2. Relevant equations
    Law of multiple proportions

    3. The attempt at a solution
    Carbon first:

    4.61/0.39 = 11.82 C/H

    Hydrogen

    0.39/4.61 = 0.08 H/C

    therefore 11.82/0.08 = 139.72 ~ 140

    masses of carbon that combine with hydrogen are in a 140:1 ratio

    Does this look right? Or have i just done the second part?

    image.jpg

    i have no idea what im doing

    Thanks for any help.
     
  2. jcsd
  3. Oct 19, 2015 #2

    BvU

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    What does this mean ? You divide C/H by H/C and get C2/H2. Is that the law of multiple proportions ?
     
  4. Oct 19, 2015 #3
    This was just a method i followed from here:

    http://chemistry.about.com/od/workedchemistryproblems/a/law-of-multiple-proportions-problem.htm

    Is this method not correct?

    As I understand it so far for AB and AB2 there are small number ratios for the B of each compound. I am unsure how to apply this to the question.

    Many thanks for the reply
     
  5. Oct 19, 2015 #4

    BvU

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    Ha, in the example, there are two substances. In one of them, there is twice as much O per C as in the other. (so one can be CO4 and the other CO2, for example -- but CO and CO2 is more probable :wink:) .

    You only have one substance, so I'm afraid you will need atomic weights to work towards a nice ratio.
     
  6. Oct 19, 2015 #5
    ahh i see thanks

    please could you expand on this a little?

    Did the 140 I got have any relevence?

    thanks again
     
  7. Oct 19, 2015 #6

    BvU

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    For two substances with different weight ratios you can divide out the atomic weights.

    Molecules are formed from atoms (or other molecules) in the ratio of small numbers of constituents. The constituents are not counted in amounts of mass but in amounts of atoms (or molecules). So you have to convert the masses to number of atoms by dividing by the atomic weights.

    [edit] And the 140 is C2 / H2 in kg2/kg2, for which I can't think of an interesting physical meaning :smile:
     
  8. Oct 19, 2015 #7
    Ahh I see, yes, ok thanks very much for your help!
     
  9. Oct 19, 2015 #8

    BvU

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    You're welcome. Interesting substance ! This ratio has puzzled mankind (well, the scientific community) for a while, until August Kekule dreamed up the solution !
     
  10. Oct 21, 2015 #9

    epenguin

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    Yes, as said, for multiple proportions law you need two compounds. If as well as data in #1 you use that in #3 you can answer the question.
     
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