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Homework Help: Mean molar mass after combustion in a combustion engine

  1. Apr 9, 2013 #1
    1. The problem statement, all variables and given/known data
    The problem is to calculate the mean molar mass of the reaktion below, which takes place in a combustion engine.

    C4H10 + z(0.79N2 + 0.21O2) → αH2O + βCO2 + γN2

    where the molar masses for the different molecules/atoms are:
    MH2O = 18, MCO2 = 44, MN2 = 28, MO2 = 32, MC = 12, MH = 1.

    The mass of the air in the cylinder before combustion is 2.39905 grams. The mass of the fuel in the cylinder is 45.0 milligrams.

    2. Relevant equations
    See above

    3. The attempt at a solution
    I have tried to balance the equation above by calculating the amount of molecules of oxygen and nitrogen in the cylinder and the amount of molecules in the fuel to get α, β and γ. And then use these values to balance the reaction equation above. The mean molar mass can be calculated by dividing the right hand side by the amount of molecules at hand.

    But, I don't get the right answer by doing this. Can anyone help me with this?

    I am very grateful for answers!

    Best regards
  2. jcsd
  3. Apr 9, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Without showing your calculations, you won't get much help.
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