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Homework Help: The ideal gas law

  1. May 15, 2008 #1
    1. The problem statement, all variables and given/known data

    When the volume of a gas is 2.81 L, the pressure is 740 Torr and the temperature is 20oC, how many moles of gas is there?

    2. Relevant equations

    PV=nRT

    3. The attempt at a solution

    740 Torr = 740*133.3 = 98.6*10^3 Pa
    Temperature in Kelvin: 293 K

    n = PV/RT = (98.6*10^3 * 2.81) / (8.3145*293) = 114 moles

    The corret answer is 0.114 moles. Does this mean that when using the ideal gas law, the pressure should be in kPa?
     
  2. jcsd
  3. May 15, 2008 #2

    chemisttree

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    What are your units for R? The pressure unit must match that.
     
  4. May 15, 2008 #3
    J/(K*mol)
     
  5. May 15, 2008 #4

    Borek

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    Staff: Mentor

    All it means is that units of data must match units of R.

    R can take any value, it all depends on the units used. For example - 10.7316 ft3·psi·°R-1·lb-mol-1. Don't ask me what lb-mol is :smile:

    Check how J/(K*mol) relates to pressure in kPa. What is Pa?
     
  6. May 15, 2008 #5

    chemisttree

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    Hint: "Oh, I thought it was in L kPa/mol K!"
     
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