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Energy content of gasoline and hydrogen comparative math question

  1. Nov 11, 2013 #1
    1. The problem statement, all variables and given/known data

    A typical car gas tank holds 15 gallons

    A) Calculate the energy content of this much gasoline
    B) Calculate the chemical energy content of 15 gallons of hydrogen gas under normal conditions (12.8 MJ/m^3) and compare with the energy content of gasoline

    2. Relevant equations

    Gasoline produces 70g of CO2 per megajoule of fuel energy

    need to qualify hydrogen in terms of energy and CO2 in terms of mass
    (3H2 → CO2)

    Burning H2 produces .475 aJ= 1.475 aJ of energy.
    CO2 has molecular weight of 44 so its mass is (44) (1.66 x 10^-27) = 7.30 x 10^ 26 kg

    3. The attempt at a solution


    I have no idea if the above information helps at all, but I really have no idea how to start this problem
     
  2. jcsd
  3. Nov 11, 2013 #2
    The question has nothing to do with CO2.
    Look up the heat or combustion or calorific power of fuels.
     
  4. Nov 11, 2013 #3
    You need to get the heat of combustion of gasoline. Maybe you could use octane as a surrogate for gasoline, or maybe you could find out the concentrations of various hydrocarbons in typical gasoline.
     
  5. Nov 11, 2013 #4

    haruspex

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  6. Nov 11, 2013 #5
    So if carbon content per gallon of gasoline is 2421, would I then do 15 gallons x 2421 grams to get 36,315 g?

    How would I go about solving part B?
     
  7. Nov 11, 2013 #6

    SteamKing

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    2421 what? Units are important to correctly working out any problem.
     
  8. Nov 11, 2013 #7
    grams
     
  9. Nov 11, 2013 #8

    haruspex

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    Yes. What mass of CO2 will that produce?
    That looks much simpler. You're told the volume and the energy per volume.
     
  10. Nov 11, 2013 #9
    I am not sure what you are saying?

    Is part A correct or do I need to do more to solve the problem?
     
  11. Nov 11, 2013 #10

    haruspex

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    Part A asks for the energy content. So far you have found the carbon content (mass). You know the relationship between the energy and the CO2 mass. So what's the missing step?
     
  12. Nov 11, 2013 #11
    I know that produces 70g of CO2. Would I use this?
     
  13. Nov 11, 2013 #12

    haruspex

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    You know what produces 70 of CO2?
     
  14. Nov 11, 2013 #13
    I know that gasoline produces 70 g of CO2 - not sure if this is relevant though?
     
  15. Nov 12, 2013 #14

    haruspex

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    "70g of CO2 per megajoule of fuel energy"

    so you know
    1. the relationship between the mass of CO2 produced and the energy produced;
    2. the relationship between the mass of a quantity of gasoline and the mass of carbon in it;
    You want
    3. the relationship between the mass of a quantity of gasoline and the energy produced;
    What one more relationship do you need to connect 1 and 2?
     
  16. Nov 12, 2013 #15
    I am not sure how to connect the two? Do I already have this info?
     
  17. Nov 12, 2013 #16

    haruspex

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    I'm sure you do. What mass of CO2 will be produced by burning 1kg of carbon? (Think about atomic masses.)
     
  18. Nov 12, 2013 #17
    So if the atomic mass of carbon is 12 and the atomic mass of oxygen is 16 what do I do with this?

    Also I have two oxygen so would I add all of these to get 44?

    The answer in the back of the book for part A is 2.2 billion J but I have no idea how this was gotten.
     
  19. Nov 12, 2013 #18

    haruspex

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    Yes. So you have, for burning gasoline:
    • 12 atomic masses C corresponds to 44 atomic masses CO2
    • 70g CO2 corresponds to 1MJ energy
    • 1 gallon gasoline corresponds to 36,315 g C
    So how many J energy correspond to 1 gallon gasoline?
     
  20. Nov 12, 2013 #19
    The answer in the back of the book is 2.2 billion J but I don't know how this was gotten. I don't know what to do with the above info
     
  21. Nov 12, 2013 #20

    haruspex

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    Each item gives you a ratio, and they connect in a chain: ratio of gasoline volume to carbon mass; ratio of carbon mass to CO2 mass; ratio of CO2 mass to energy. Just connect them up.
     
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