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Percent composition

  1. May 14, 2009 #1
    1. The problem statement, all variables and given/known data
    An herbicide is found to contain only C, H, N, and Cl. The complete combustion of a 100.0-mg sample of the herbicide in excess oxygen produces 83.16 mL of CO2 and 73.30 mL of H2O vapor at STP. A separate analysis shows that the sample also contains 16.44 mg of Cl.

    Determine the percent composition of the substance.


    2. Relevant equations

    pV = nRT

    3. The attempt at a solution

    moles of Cl = .01644/35.453
     
  2. jcsd
  3. May 14, 2009 #2

    Borek

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    Staff: Mentor

    You don't need moles of chlorine. Percent composition means mass/mass.
     
  4. May 14, 2009 #3
    So how do I get the mass of the other elements?
     
  5. May 14, 2009 #4

    Borek

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    Staff: Mentor

    From moles. In the case of chlorine mass was given straight away.
     
  6. May 14, 2009 #5
    How do I figure out the number of moles for the other elements?
     
  7. May 14, 2009 #6

    Borek

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    Using equation that you already posted.

    Thats enough spoonfeeding, you either start to think on your own, or I am not going to help any further.
     
  8. May 14, 2009 #7
    moles of CO2 = pV/RT = 0.08316/(0.0821 * 273) = 0.003710297 moles
    Moles of C = (12/44) * 0.003710297 = 0.00101199 moles
    mg of C = (0.00101199)*(12000) = 12.14mg

    moles of H2O = pV/RT = 0.07330/(0.0821 * 273) = 0.00327038 moles
    moles of H = (2/18) * 0.00327038 = 0.000363376 moles
    mg of H = (0.000363376)*(1000) = 0.363375516 mg

    mg of N = 100mg - 0.363375516 mg - 12.14mg - 16.44mg = 71.05 mg

    So the % composition is
    C 12.14%
    H 0.3634%
    N 71.05%
    Cl 16.44%

    Is that right?
     
  9. May 14, 2009 #8

    symbolipoint

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    The method looks good, ( I did not check for arithmetic computation mistakes ).
     
  10. May 14, 2009 #9

    Borek

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    Staff: Mentor

    No. 1 mole of CO2 contains 1 mole of C.
     
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