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Homework Help: Mole fraction of gas in a mixture

  1. Mar 17, 2015 #1
    1. The problem statement, all variables and given/known data
    At 1 atm and 0° C, a 5.04 L mixture of methane (CH4) and propane(C3H8) was burned producing 20.9 g of CO2. Assume complete combustion.
    1. How many moles total of methane and propane were present before combustion?

    2. How many moles of carbon dioxide were present after the reactoin?

    3. What was the mole fraction of each gas in the mixture?

    2. Relevant equations
    PV = nRT

    3. The attempt at a solution
    1. PV = nRT
    n = 0.224743 moles of mixture

    2. I converted 20.9 g CO2 to moles giving:
    0.475 mol of CO2

    3. This is where I'm stuck.
    First I balanced the combustion of the mixture:
    CH4 + C3H8 + 7O2 -> 4CO2 + 6H20
    Then all I have is that 0.224743 mole of the mixture. I don't know how to find the number of moles of CH4 and C3H8 each so I can find the mole fraction.
  2. jcsd
  3. Mar 17, 2015 #2

    Quantum Defect

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    Homework Helper
    Gold Member

    This is where you use stoichiometry. From the moles of CO2, can you figure out how many moles C there are? Once you kno wthat you can write:

    Moles C = x moles CH4 + y moles C3H8.

    Where, "x" must have units of (moles C)/(moles CH4) to give you the right units. For any quantity of CH4, how many moles C are there for every mole CH4?
    Same thing goes for "y" -- it must have units of (moles C)/(moles C3H8). How many moles C for every mole C3H8?
  4. Mar 17, 2015 #3

    Suraj M

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    Gold Member

    When? If it was at the beginning then form 2 equations, you'll get one from total no. of moles. and the other from stoichiometry, try getting them first, as Quantum defect suggests, above
    Last edited: Mar 17, 2015
  5. Mar 17, 2015 #4


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    Staff: Mentor

    That's not combustion of the mixture. What you wrote describes - at best - what happens when you burn equimolar mixture, but your mixture is not necessarily equimolar. Each gas reacts separately, according to its own stoichiometry of combustion (hence you have two reaction equations, not one).
  6. Mar 17, 2015 #5
    How does the total number of moles of CO2 in the product relate to the total number of moles of C in the reactants?

    Take as a basis 0.225 moles of methane and propane, and 0.475 moles of CO2 in the product. Let x = moles of methane in the reactants and y = moles of propane in reactants. In terms of x and y, what is their relation to the 0.225 moles? In terms of x and y, what is their relation to the 0.475 moles of CO2?

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