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Homework Help: HELP Pressure Change for equation without volume

  1. Jan 21, 2010 #1
    HELP!!! Pressure Change for equation without volume

    1. The problem statement, all variables and given/known data

    A tank is pressurized with 5.0 atm of N2 and 10.0 atm of H2. Ammonia (NH3) is formed. When the pressure finally remains constant, indicating that the reaction has proceeded as far as it will go, the partial pressure of ammonia is 3.2 atm. What is the total pressure in the tank assuming that neither the temperature nor the volume of the container have changed?

    2. Relevant equations


    3. The attempt at a solution

    I don't know how to go about this without using volume or number of moles. Is there another equation I should use??
    Any help would be greatly appreciated
  2. jcsd
  3. Jan 21, 2010 #2


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    Re: HELP!!! Pressure Change for equation without volume

    Assume the tank has volume V. You will see that the V cancels out in the end. (alternatively if you like using numbers instead of variables, just use an arbitrary volume like 1L, plug in the numbers and see what the answer is).
  4. Jan 21, 2010 #3
    Re: HELP!!! Pressure Change for equation without volume

    I agree with Ygg.. Assume volume and temperature are the same at beginning and end. Then the pressure is proportional to moles. The equation is N2+3H2 -> 2NH3. Assume this has an extent of x (x moles of N2 reacted). Starting moles N2=5, H2=10. Ending moles N2=5-x, H2=10-3x, NH3=2x. Total pressure at end = sum of three partials = 15-2x.
    The problem gives you NH3= 3.2 atm at end = 2x. Total pressure at end= you solve it.
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