Chemistry Help!

Discussion in 'Community Discussion' started by TSE, Feb 18, 2009.

  1. TSE macrumors 68030

    Joined:
    Jun 25, 2007
    Location:
    St. Paul, Minnesota
    #1
    Hey guys, we are studying pressures, temperatures, etc. right now... and on my homework tonight I have a certain question I don't know how to answer...

    The gas in a closed container has a pressure of 3.00 multiplied by 102 kPa at 30°C (303 K). What will the pressure be if the temperature is lowered to -172°C?

    How do I do this problem and what is the answer? Thanks ahead of time!
     
  2. mknawabi macrumors 6502

    mknawabi

    Joined:
    Feb 5, 2006
    Location:
    Irvine, CA
    #2
    PV = nRT
    (pressure)*(volume) = (#moles)(universal gas constant)(temperature in kelvin)

    plug in for pressure. don't count volume since it is a constant anyway. convert -172 to kelvin... do your algebra
     
  3. dukebound85 macrumors P6

    dukebound85

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    5045 feet above sea level
    #3
    make sure you are using the right units for pressure. depending on how you have the mol ratio
     
  4. SLC Flyfishing Suspended

    SLC Flyfishing

    Joined:
    Nov 19, 2007
    Location:
    Portland, OR
    #4
    You need to specify what the 3 in your pressure means. I'm assuming 3 atm. but if I'm not sure then I can't really help.

    Knowing what unit you'll indicate the pressure with is of utmost importance when picking a value for the ideal gas constant.

    SLC
     
  5. CorvusCamenarum macrumors 65816

    CorvusCamenarum

    Joined:
    Dec 16, 2004
    Location:
    Birmingham, AL
    #5
    I haven't had chemistry since my undergraduate days, but wouldn't the combined gas law be more effective here?

    P1V1/T1 = P2V2/T2
    As volume is constant, strike it from the equation: P1/T1 = P2/T2
    P1T2 = P2T1
    P2 = P1T2/T1
    P2 = (P1*101)/303

    (I'm a tad confused on the original pressure as well)

    ?
     
  6. mknawabi macrumors 6502

    mknawabi

    Joined:
    Feb 5, 2006
    Location:
    Irvine, CA
    #6
    Actually, you are most definitely right.
     
  7. SLC Flyfishing Suspended

    SLC Flyfishing

    Joined:
    Nov 19, 2007
    Location:
    Portland, OR
    #7
    That would be more efficient I suppose!

    It's probably bad that I only remember the combined gas law when these sorts of things come up. Not that it's not able to deliver the correct solution, but still.........

    SLC
     
  8. eddietr macrumors 6502a

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    Oct 29, 2006
    Location:
    Virginia
    #8
    The original pressure is 3.00 times 102 kPa. In other words, 306 kPa.
     
  9. CorvusCamenarum macrumors 65816

    CorvusCamenarum

    Joined:
    Dec 16, 2004
    Location:
    Birmingham, AL
    #9
    Assuming you're correct, solving for P2 gives us 102 kPa.
     
  10. eddietr macrumors 6502a

    Joined:
    Oct 29, 2006
    Location:
    Virginia
    #10
    Sounds about right to me. And the equation you posted will hopefully help the OP see how you got that.

    Intuitively, if :
    1. You hold n,R, and V constant (closed container)
    2. PV=nRT
    3. you reduce the T to about 1/3 of the original condition.

    then the pressure must be about 1/3 of the original also.
     
  11. TSE thread starter macrumors 68030

    Joined:
    Jun 25, 2007
    Location:
    St. Paul, Minnesota
    #11
    Thanks guys, I will show the professor tomorrow. Thank you for your help! :eek:
     

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