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Homework Help: Pressure of gas

  1. May 18, 2015 #1
    1. The problem statement, all variables and given/known data
    Gas occupies 2m^3 at a pressure of 100kPa. What is the pressure when the volume is 1.5m^3 and the temprature is unchanged?

    2. Relevant equations

    3. The attempt at a solution
    As a logic conclusion i took 1.5/2x100 and got 75 but 175 is not the answer.. The answer is 133kPa.
    I also tried the p=nRT/V.
    22.4l=1mol so 1500/22.4x1 is 66.96mol. (66.96x22.4x0)/1500=0
    Can someone please tell me how to calculate this type of problem?

    Thank you.
  2. jcsd
  3. May 18, 2015 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Well, your "logic conclusion" got warped somehow.

    If you take a certain volume of gas and then shrink that volume, keeping the amount of gas and its temperature constant, what do you think happens to the pressure of the gas confined into that new, smaller volume?

    If you analyze the relation PV = nRT, n is the same, R is a constant, and T is the same. So what happens to PV if V gets smaller?
  4. May 18, 2015 #3
    Always check your units to make sure that what you are calculating makes sense.
    You are analyzing the gas in two states.
    Start with what you know. What is the pressure and volume of the gas in the first state? What do you know about the second state? I would encourage you to use subscripts. P1 is the pressure of the gas in state 1. P2 is the pressure of the gas in state 2. V1 is the volume of the gas in state 1. etc.
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