Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Confusion with the gas constant!

  1. Oct 6, 2006 #1
    For the ideal gas law and van der waals eq'n, R can be a) 8.3145J/(k*mol) or b) 0.082058(L*atm)/(k*mol) . But if I use the first value of R, and try to solve for P my value always comes out as about 1000 smaller than it should be in Pa(ie/ it should be 2000 but it gives me 2). If I use the 2nd value, my value comes out in atm which is okay. But I cant use this value in another eq'n I need, so I don't want to use it. Is the ideal gas law supposed to be in kPa?
     
  2. jcsd
  3. Oct 6, 2006 #2
    hey,

    i think since the gas constant is being used to calculate pressure...it should be the .082058(L*atm)/(k*mol)

    the other one is for thermodynamics...it's units include the energy (J)

    correct me if im wrong
     
  4. Oct 6, 2006 #3
    gas constants

    the gas constant is either 0.08206L*atm/(K*mol) or 8.314L*kPa/(K*mol)

    you should be able to use either or. i personally like to use kPa, but if you want to use atm, you could always convert from atm to kPa and the other way around using the equality 1atm = 101.3kPa.
     
  5. Oct 6, 2006 #4
    i disagree because one gas constant is used for thermo problems (heat and energy) and the other is for pressure probs...There is a definite difference between the two numbers
     
  6. Oct 6, 2006 #5

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    WRONG!!!! Units of the gas constant are energy per amount of substance per unit temperature; pressure x volume = energy. So long as one is consistent with units throughout a problem, one gets the "same" answer regardless of "which" gas constant one uses.
     
  7. Oct 6, 2006 #6
    well, yeah thats what i meant when i posted previous response...I was definitely implying units of measurements!--if they give a pressure (atm, kpa, etc) or energy (J, kj...) keep consistent by using the appropriate gas constant

    thinking back at my prev response...i can see it's sorta half wrong because i didnt set a stipulation for using a gas constant--my bad :redface: :wink:...
    they're not different because they're both gas constants
    PS: i havent had much experience with vander's law, but ive definitely plugged things into it...
     
    Last edited: Oct 6, 2006
  8. Oct 7, 2006 #7

    Borek

    User Avatar

    Staff: Mentor

    0.08205783 L*atm/(K*mol)
    8.314472 kPa*dm3/(K*mol)
    8,314472 L*kPa/(K*mol)
    8.314472 J/(mol*K)
    62,3637 L*mmHg/(K*mol)
    83,14472 L*mbar/(K*mol)
    1.987216 cal/(K*mol)

    or something :wink:
     
  9. Oct 7, 2006 #8
    pwned


    or something...
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook