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Mass Relationships In Chemical Reactions

  1. Oct 5, 2008 #1
    1. The problem statement, all variables and given/known data

    Styrene is a Hydrocarbon. If 0.438g of Styrene is burned in Oxygen and produces 1.481g of CO2 and 0.303g of H20, what is the empirical formula of styrene?

    2. Relevant equations



    3. The attempt at a solution

    No. of moles of CO2 - 1.481g / 44.0107g/mol = 0.0337mol
    No. of moles of C - 0.0337mol
    No. of moles of H2O - 0.303g / 18.016g/mol = 0.0168mol
    No. of moles of H - 2 x 0.0168mol = 0.0336mol
    Mole ratio of C and H - 0.0337 : 0.0336
    1 : 1

    Is this ratio the empirical formula of Styrene?
     
  2. jcsd
  3. Oct 5, 2008 #2
    I got the same answer when I tried it.
     
  4. Oct 5, 2008 #3
    Thank you for confirming my answer. By the way, does the coefficient of each compound in a balanced equation affect the number of moles of that substance?
    Example:
    There is an equation where there is H2O and the question says that the number of moles of H2O is 4 moles. If I balance the equation and the coeffcient of H2O happens to be 2, does that mean that the no. of moles of becomes 8 moles?

    Thanks for answering my first question again.
     
  5. Oct 5, 2008 #4
    You're welcome!

    To answer your second question- the moles of H2O could be affected, but it would depend on what the question is asking you to do. If you post the entire question, I might be of more assistance.
     
  6. Oct 6, 2008 #5
    Thanks for responding but I managed to solve that problem myself but faced a new one. Please help me for this one too. Thanks alot.

    Element Z reacts with Hydrogen to produce a pure sample of Z2H5 in an experiment, it is found that 1.0g of Z produces 2.7896g of Z2H5 . How do I find the molar mass of Z?

    Is this possible?
    2.7896 - 1 = 1.7896 (Mass of Oxygen)
    1.7896g / 3 x 1.008g/mol = 0.5918mol (Of O3)
    Mole ratio of Z2H5 - 2:5
    0.5918 / 5 x 2 = 0.23672mol (No. of moles of X)
    1g / 0.23672mol = 4.22441g/mol.

    Is it correct to take the mass of Oxygen present / The molar mass of O3 to get the number of moles of O3? If so, is it correct to take the number of moles of O5 / 5 x 2 or take the number of moles of O / 5 x 2?

    Thanks again for helping!
     
  7. Oct 6, 2008 #6
    I was looking at a problem like this in another thread. I am only in my first quarter of AP Chem and we have not worked with problems like this yet. Here is the thread https://www.physicsforums.com/showthread.php?t=261923. These problems have me fired up now! I'm going to class early today so I can have my teacher show me how to do them! :]
    I'm sorry I could not be of greater assistance.
     
    Last edited: Oct 6, 2008
  8. Oct 6, 2008 #7
    Kk, pls show me the solution when you are done. Thank you.
     
  9. Oct 6, 2008 #8
    In the first part of your problem you say that you are looking for Z2H5, but then in your work you are using O. I am not quite sure where you are getting that from.

    My chem teacher briefly showed me how to do it and did not have time to check my answer. I am 99% positive that I got the wrong answer though. It came out to be some thing like 2x+50395g/1383867447moles lol. I will ask her about it again tomorrow. I am sorry if this is due before then.
     
  10. Oct 6, 2008 #9

    Borek

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    Staff: Mentor

    Learn how to read chemical reactions - I think it should answer your question.
     
  11. Oct 6, 2008 #10

    Borek

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    Staff: Mentor

    Your post is so inconsistent because of this hydrogen/oxygen thing that it is hard to help. You are partially wrong and partially OK.
     
    Last edited: Oct 6, 2008
  12. Oct 7, 2008 #11
    I think I kinda got mixed up with Oxygen and Hydrogen. This is what I meant;

    Element Z reacts with Hydrogen to produce a pure sample of Z2H5 in an experiment, it is found that 1.0g of Z produces 2.7896g of Z2H5. How do I find the molar mass of Z?

    Is this possible?
    2.7896 - 1 = 1.7896 (Mass of Hydrogen)
    1.7896g / 5 x 1.008g/mol = 0.3551mol (Of H)
    Mole ratio of Z2H5 - 2:5
    0.3551 x (2/5) = 0.14204mol (No. of moles of X)
    1g / 0.14204mol = 7.0403g/mol.

    Is it correct to take the mass of Hydrogen present divided by the molar mass of H5, to get the number of moles of H5? If so, is it correct to take the number of moles of H5 x (2/5) or take the number of moles of H x (2/5)?

    Thanks for helping again!
     
  13. Oct 7, 2008 #12

    Borek

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    Staff: Mentor

    OK

    No. 1.7869/1.008 = 1.773 mol of H. What you calculated would be 0.3551 mol of H5, whatever it means. Not much.

    OK

    OK, although numbers are incorrect because you were wrong earlier.

    OK, although numbers are incorrect because you were wrong earlier.

    As long as you are consistent you can calculate it whichever way want. But you can't go for number of moles of H5 and then multiply it by 2/5 - you have to multiply it by 2. At the same time if you have number of moles of H you have to multiply it by 2/5, but you can't by 2.

    My advice: forget about the H5 approach. It gives correct results when done properly, but you may have a hard time trying to convince your teachers that you were on the right track.
     
  14. Oct 8, 2008 #13
    Is it true to say that if I take the mass of an element, X5 for example, and divide it by the molar mass of X times the subscript(5), I will get the number of moles of X5?
     
  15. Oct 8, 2008 #14

    Borek

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    Staff: Mentor

    Yes. That's what is routinely done for diatomics, like O2.

    Just try to be precise, when you write

    it raises a brow - no such element as X5. X5 is a molecule. Element is X.
     
    Last edited: Oct 8, 2008
  16. Oct 8, 2008 #15
    Thanks very much. I've finally got it.
     
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