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Homework Help: Ideal Gas Equation Help

  1. Jul 24, 2007 #1
    Im having problems with my ideal gas equations. I am able to do the actual equation but when I comes to the answer, I have difficuly understanding.
    Here is an example.
    If i want to discover the pressure...


    Now the answer my calculator gets is 24.486704.

    Now, I am not sure if this is correct, it doesnt look right to me, the pressure is too low. Do i have to move the decimal point? Is the pressure really 244.86704?

    Help me out please!
  2. jcsd
  3. Jul 24, 2007 #2


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    What units is that pressure in?

    It is good practice to always carry the units associated with physical numbers. You can use the units to verify answers. If you do not get the right units on a result, then you have a error some where.
  4. Jul 24, 2007 #3


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    It's difficult to have good intuition about pressure.
    Do you know about scientific notation - where you write numbers as powers of ten?
    Then you can quickly check your working to an order of magnitude.
    V=1e3 n=8e2 r=8e-2 t=3.7e2
    pv=nrt so p =nrt/v
    = 8e2 * 8e-2 * 3.7e2 / 1e3 = 64*3.7e-1 = 64*0.3 = 24

    When doing calculations, always check the units are the same and do an order of magnitude estimate.

    Hint - what units is your volume in?
  5. Jul 24, 2007 #4


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    In order to get an intuition for the ideal has laws it's very helpful to have the "atom picture" in your head. Meaning that pressure is the combined force of all the atoms pushing against the box by random collisions with it. Make the box smaller - the collisions are more frequent. Raise the temperture - the atoms move faster and hit harder. Fenyman explains it well in "Lectures on Physics".
  6. Jul 24, 2007 #5


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    I meant it's difficult to picture what 1 Pascals means in the same way that you can picture 1kg or 1m, it's less tangible.

    Is 800moles in 1000l a lot of material or very little - without working out the answer all you can do is compare it to atmospheric pressure.
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