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Homework Help: Molecular formula of hydrocarbon

  1. Jan 15, 2008 #1
    1. The problem statement, all variables and given/known data

    0.1 dm^3 of a gaseous hydrocarbon may require 0.45 dm^3 of oxygen for a complete combustion to give 300 cm^3 of Carbon dioxide. All volumes are measured in the same environment. Calculate the molecular formula of the hydrocarbon.

    3. The attempt at a solution

    i tried getting the gdc of 10, 30 and 45 =90. and the have like

    9 CxHy + 2 O2 --> 3 CO2 + H2O

    but I cannot go further than this. please help. Thanks.
  2. jcsd
  3. Jan 15, 2008 #2
    as for the formula you have taken.. i don't understand why you've taken the GCD, but it is fundamentally wrong as the oxygen is not balanced on both sides [and neither is carbon or hydrogen].

    What you need is a equation which is prefixed and suffixed by variables on both the sides to get you a linear equation which you can then solve...
  4. Jan 15, 2008 #3


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    Can you think of a way to calculate the amount of carbon in CO2 and hydrogen in H2O? You have a good start here:
    I've removed the prefixes you had (the '9', '2' and '3'). Can you solve it in terms of 'x' and 'y'?
  5. May 26, 2009 #4
    CxHy + (x+y/4)O2 --> xCO2 +y/2H2O

    thats the correct answer from ccea spec for alevel chemistry =]
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