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Partial Pressure Calcs.

  1. Jun 5, 2005 #1
    Question 1: 5L of nitrogen at 100kPa and 298K and 5L of oxygen at 200kPa are pumped into a evacuated 50L cylinder. Temperature remains constant, what is the total pressure in the cylinder?

    Correct Answer: 30kPa
    My answer: 300 kPa

    Question 2: A 10L flask containing nitrogen at 500mmHg is connected via a closed tap to a 4L flask containing oxygen at 800mmHg pressure, if the tap is opened what will be the final pressure assuming temperature remains constant?

    Answer: 586mmHg
    My answer: 1300mmHg

    Formulas I used:

    pV = nRT (to find mols)
    PA = (nA/nA+nB) x PT

    How did they get these answers...cos I can't seem to get them..?? Any help would be appreciated...
  2. jcsd
  3. Jun 5, 2005 #2


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    What are the volumes given in the questions?
  4. Jun 5, 2005 #3
    what do u mean? I gave the volumes... :confused:
  5. Jun 5, 2005 #4
    So what is it exactly that you are having a hard time with? Let's go through #2:

    [tex]n_A = \frac{P_AV_A}{RT} = \frac{(500 mm Hg)(10 L)}{RT}[/tex]

    and for the other flask:

    [tex]n_B = \frac{P_BV_B}{RT} = \frac{(800 mm Hg)(4 L)}{RT}[/tex]

    so when the valve is opened, the total # of molecules of gas in the system will be:

    [tex]n = n_A + n_B[/tex]

    so lets use the ideal gas law for the whole system:

    [tex]P_{A+B} = \frac{(n_A + n_B)(RT)}{V_{A+B}} = \frac{(500 mm Hg)(10 L) + (800 mm Hg)(4 L)}{RT} \frac{(RT)}{V_{A+B}}{[/tex]

    notice that RT drops out completely, and we are left with:

    [tex]P_{A+B} = \frac{(500 mm Hg)(10 L) + (800 mm HG)(4 L)}{14 L} = 586 mm Hg[/tex]

    go forth and do likewise..
    Last edited: Jun 5, 2005
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