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Homework Help: Gas law

  1. Mar 22, 2010 #1
    1. The problem statement, all variables and given/known data

    A spherical balloon is inflated to a diameter of 36 cm. Assuming that the gas in the balloon is of atmospheric pressure (101.3 kPa) and is at a temperature of 20°C.

    (a) How many moles of gas are in the balloon?

    (b) If 21% of the molecules in the balloon are oxygen molecules, how many oxygen molecules are in the balloon?

    3. The attempt at a solution

    (a) I want to use the formula [tex]PV=nRT[/tex]

    Volume is [tex]v=\frac{4}{3} \pi (0.18)^2 = 0.1357[/tex]

    R is the constant 8.314

    So [tex](101.3)(0.1357)=n8.314(20)[/tex]

    Now if I solve for n I get [tex]n= {13.74}{166.28} = 0.082[/tex]

    This is wrong because the correct answer must be 1.02. Do I need to convert the temprature to Kelvin/absolute temprature? ...because I tried that too & it didn't work.
  2. jcsd
  3. Mar 22, 2010 #2


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    Given pressure is in kilopascal.
    Temperature must be in Kelvin.
    Last edited: Mar 22, 2010
  4. Mar 22, 2010 #3


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    Temperature should be in kelvin
  5. Mar 22, 2010 #4


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    Your temperature should be in Kelvin (K) and the pressure in Pa
  6. Mar 23, 2010 #5
    Also, you squared your radii instead of cubing it.
  7. Mar 23, 2010 #6
    Okay 20 degrees is 293.15 in Kelvin, and I converted the pressure to pascals. So my equation becomes


    n= 5.64

    But why is this still not the right answer? :confused:
  8. Mar 23, 2010 #7


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    Check the volume of the gas.
    V = 4/3*π*r^3
  9. Mar 23, 2010 #8
    = [. I said that earlier. I'm sad now.
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