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Homework Help: Calculate the mass of propane that has been used

  1. May 28, 2010 #1
    1. The problem statement, all variables and given/known data

    A cylinder 1.00 m tall with an inside diameter of 0.120 m is used to hold propane gas (molar mass: 44.1 g.mol-1) for use in a barbeque. It is initially filled with a gas until the gauge pressure is 1.30 x 10^7 Pa and the temperature is 22.0⁰C. The temperature of the gas remains constant as it is partially emptied out of the tank, until the gauge pressure is 2.50 x 10^5 Pa. Calculate the mass of propane that has been used.

    2. Relevant equations

    1.. pV = nRT
    2.. p1V1 / T1 = p2V2 / T2

    3. The attempt at a solution

    I am really wrong here but just need a guiding hand...

    The volume of the cylinder is 0.11309733m^3
    From Eq 1 I get the number of moles = 60 then total mass = 60 x 44.1 = 2643.64g
    Then did this again for pressure 2 (2.5 x 10^5 Pa) to get = 50.84g
    Difference being 2592.8g of gas used but this seems large.

    I then looked at Eq 2 and I got V2 = 0.588m^3
    Not really sure where to go with that....
     
  2. jcsd
  3. May 28, 2010 #2

    Borek

    User Avatar

    Staff: Mentor

    Second equation assumes number of moles of gas have not changed - so it is of no use, some of the gas was burnt.

    Your first approach looks OK, just check your math again. Mass difference is much larger than you think. Seems like you have problems with position of the decimal point.
     
  4. May 28, 2010 #3
    Hi Borek,

    I typed that wrong! It should be 0.011309733m^3

    The final value of 2592.8g was calculate from that figure. So is this what you get?
     
  5. May 28, 2010 #4

    Borek

    User Avatar

    Staff: Mentor

    Obviously when it comes to calculating volume of cylinder I am not better than you :grumpy:

    Your result is OK, just watch significant figures.

    --
     
  6. May 28, 2010 #5
    LOL, thanks heaps Borek.
     
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