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Homework Help: Please help to calculate pressure

  1. Mar 4, 2009 #1
    1. The problem statement, all variables and given/known data

    Hi I am an Engineering student and i have to do an analysis on a closed system vessel that is going to be heated up.And before i start the experiment i want to an analysis or a safety factor for this experiment.

    I am going to have a closed system vessel, lets say the volume is going to be 3 m^3. and surface area of 3m^2.
    Initially i am going to put an argon gas inside the closed vessel when the vessel is still at room temperature (25C) and then heat it up until it reach 500Celsius.(T=773K)

    2. Relevant equations

    Pv=nRT, and density=m/V

    3. The attempt at a solution

    and gas volume is always the same with the medium right? so the argon gas volume on a 3m^3 vessel will be 3m^3 as well right?

    but isnt it that the volume of the gas depends on the temperature and the pressure?

    or can i just calculate the mass by using the density of the argon gas at 25C and find the mass or the mass is going to be different at 500C?

    Sorry about a lot of confusion in my posting, i am really confused and do need help. Thank you
  2. jcsd
  3. Mar 4, 2009 #2


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    Actually a 3 m^3 volume must have a surface area of at least 10 m^2, but that isn't really important for this problem.

    Those 3 are all related to each other, through PV=nRT.

    Since V is a constant (in this situation), what can you say about the ratio P/T?

    p.s welcome to PF :smile:
  4. Mar 4, 2009 #3
    Thnx redbelly98!

    ohhh so the volume stays the same? well :) i kno its a stupid question, its just that the volume tend to increase with the temperature right?
    And it is true right for Argon gas i can just use ideal gas equation, or should i go with the non-ideal gas formula?

    Thanks again!
  5. Mar 5, 2009 #4


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    At room temperature and 1 atm pressure, argon is close to an ideal gas.

    To calculate the final pressure, you can try both ideal and real gas solutions and compare them.

    Since n/V is fixed, and can be calculated at room temp & 1 atm, the real gas calculation is not as complicated as you might think.
  6. Mar 5, 2009 #5
    Yea i have tried both real gas and ideal gas equation and both of them almost the same, took me so long to find the van der waals constant+equation. Thanks Redbelly!
    and for both of calculation i used the density of argon at STP(1atm and 25C) which is 1.449kg/m3 is that okay?

    so it will be like this

    PV=nRT --> n = m/M and Rspecific = R/M so PV=mRspecific T --> P = density*Rspecific*T

    so P = density * Rspecific * Temp
    = 1.449 kg/m3 *0.208 KJ/kgK *773K
    = 232.2 KPa = 2.29 atm (I have tried to calculate it using Vdwaals equation and it got me almost the same result. )

    is that correct?
    but it is just weird to use the density of STP value to calculate in 500C temperature. Because i know in h2O there is different value of density that you have to use in different type of temperature and pressure, this confuses me, like do i need to find the exact value of density of argon at 500C or i can just use the SPT one?
    but in the other hand density is mass/volume and in this case mass and volume stays the same since its a closed vessel, so may be i could use the same density value, is that the reason?
    I am sorry if i confuse you with my 100000questions, is just that i want to learn+do this right :)

    Please help&enlighten me..Thanks again Redbelly!! :)
    Last edited: Mar 5, 2009
  7. Mar 5, 2009 #6


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    You're making it more complicated than necessary.

    PV = nRT

    Volume and # of moles are constant. So what is the relation between P and T?
  8. Mar 5, 2009 #7
    directly proportional?
    k thanks redbelly! i think i got it! thnx
  9. Mar 5, 2009 #8


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    Yes, exactly.
    You're welcome :smile:
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