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Homework Help: Compound contains only carbon, hydrogen, nitrogen, and oxygen.

  1. Aug 14, 2009 #1
    1. The problem statement, all variables and given/known data
    A compound contains only carbon, hydrogen, nitrogen, and oxygen. Combustion of 0.157 g of the compound produced 0.213 g CO2 and 0.0310 g H2O. In another experiment, it is found that 0.103 g of the compound produces 0.0230 g NH3. What is the empirical formula of the compound? Hint: Combustion involves reacting with excess O2. Assume that all the carbon ends up in CO2 and all the hydrogen ends up in H2O. Also assume that all the nitrogen ends up in the Nh3 in the second experiment.

    2. Relevant equations

    3. The attempt at a solution
    What I have is
    ___+ O2 --> CO2 + H2O
    I got stuck after this. I wonder how a compound that contains C, H, N and O yields only NH3. where did C and O go?
    Please help me. Thank you so much
  2. jcsd
  3. Aug 14, 2009 #2


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    Gold Member

    Re: Stoichiometry

    There are two experiments. In one experiment you determine the amount of C and H for a given sample size. This can be expressed in percentage. For example 4.4 g CO2 in the first experiment can be expressed as 1.2 g C (elemental). If the original sample that was analyzed in that experiment was 10 g, you have a sample that is composed of 12% C (1.2g/10g*100). Same applies to the hydrogen and the ammonia determined in the second reaction. Once you have those quantities expressed in percentages, add them together. They should sum to 100%... or should they?

    You will find that they won't.

    What is missing? Why couldn't "it" (that which is missing) be determined in a combustion experiment or the Nitrogen analysis (probably a keldahl analysis)?
  4. Aug 14, 2009 #3
    Re: Stoichiometry

    The question did not specify that NH3 is the only product; that is clearly impossible. All that it mentioned was that a certain amount of NH3 was produced (along with other product(s))
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