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Pressure/Volume question. A2 Physics

  1. Jun 25, 2011 #1
    1. The problem statement, all variables and given/known data

    A hand pump was used to raise the pressure of the air in a flask of volume 1.2 x 10^-4 m^3, without and then with powder in the flask.
    - Without powder, pressure increased from 110kPa to 135kPa
    - With 0.038kg of powder, pressure increased from 100kPa to 141kPa

    1) Show that the volume of air in the hand pump initially was 2.7 x 10^-5 m^3.
    2) Calculate the volume and the density of the powder.


    2. Relevant equations
    pV=nRT

    P=pressure V=volume n=#of moles R=gas constant T=temp


    3. The attempt at a solution
    For the first part, I found the increase in pressure factor to be 1.23, multiplied this to the volume, then took away the original volume which got me an answer of 2.76x10^-5... I wasn't sure if that was the right method to take so I didn't want to start the second part without fully understanding the first.
     
  2. jcsd
  3. Jun 25, 2011 #2
    I have solved this now. If anybody would like to know the solution for future reference I will be happy to help.
     
  4. Jun 25, 2011 #3

    cepheid

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    Staff Emeritus
    Science Advisor
    Gold Member

    For the first part, since the volume of the flask is constant, when you add gas using the pump, it seems to me that both the pressure and temperature will go up. Since you know the ratio of pressures, it seems to me that the best you can do using the ideal gas law is to say that

    n2T2 = 1.23 n1T1

    Without knowing the ambient temperature T1, I don't see how you would solve for the new amount of gas in the flask n2. I think you need to know this, because you need to then use Avogadro's law to compute how much volume the added amount of gas (n2-n1) would have occupied at ambient temperature. Was T1 given in the problem, or am I just missing something?
     
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