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Homework Help: Mass of used hydrogen in an isothermal process

  1. Jun 14, 2012 #1

    Uku

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    1. The problem statement, all variables and given/known data

    I have hydrogen in a 12 L tank, at T=15 C. Some of it is used, the T = const. and Δp=0.4 MPa,
    the molar mass of hydrogen is M=2*10^-3 J/(mol*K). Find the mass of the used hydrogen.

    2. Relevant equations

    Am I wrong in simply using:

    [itex]\Delta m=\frac{M}{RT}V\Delta p[/itex] ?

    That gives roughly 4 kg's..

    Thanks,
    U.
     
  2. jcsd
  3. Jun 14, 2012 #2
    Hi Uku,

    The equation you have used is appropriate. You could have gone wrong in the units, though.
     
  4. Jun 14, 2012 #3

    Uku

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    By "could have" you mean you put in the numbers and got a different answer?
     
  5. Jun 15, 2012 #4
    No, I didn't put in the numbers. I was just confirming the equation use :smile:

    Though, 4 kg does seem a bit too much....:rolleyes:
     
  6. Jun 15, 2012 #5

    ehild

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    The answer is different from yours. What number did you use for the volume? And the molar mass is 2*10-3 kg/mol, not J/(mol*K):tongue:
     
  7. Jun 15, 2012 #6

    Uku

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    Well the pressure differential is 0.4 MPa..
     
  8. Jun 15, 2012 #7

    ehild

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    The pressure is all right, but what number have you plugged in for volume?


    ehild
     
  9. Jun 16, 2012 #8

    Uku

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    0.012 m3
     
  10. Jun 16, 2012 #9

    ehild

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    Well, then plugging in all data,
    Δm=(2*10-3*0.012*0.4*10-6)/(8.3145*288)=9.6/2394 how can it be 4 kg? :confused:

    ehild
     
  11. Jun 16, 2012 #10

    Uku

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    I messed up on R, since I wrote [itex]R=8.314*10^{-3}\frac{kJ}{K}[/itex] when trying to write R in terms of kJ.
     
  12. Jun 16, 2012 #11

    ehild

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    That I did not guess. :surprised Why in kJ-s?

    But You got the correct result at the end now? :biggrin:

    ehild
     
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