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Finding normal boiling point given pressure and enthalpy.

  1. Sep 3, 2009 #1
    1. The problem statement, all variables and given/known data
    Given the following data, determine the heat of vaporization and normal boiling point of ammonia
    Temp (K):
    200
    210
    220
    230
    235

    Pressure (respectively)(torr):
    65.3
    134.3
    255.7
    456.0
    597.0



    2. Relevant equations
    ln(p2/p1)=-deltaH/R(1/t2-1/t1)


    3. The attempt at a solution

    I have calculated using the first two sets of data that deltaH should be 25.179kJ/mol. The books answer within 1kJ/mol, so I'm assuming that they just used different data to come up with it. Anyways, I'm having a problem on the second part of the question where it asks to come up with a normal boiling point. The normal boiling point is when the vapor pressure equals the external pressure, right? But I have no variable in this equation that will give me the normal boiling point. Do I use this equation for both portions to the question? I'm confused. I should be able to solve the problem, just need a little boost please.
     
  2. jcsd
  3. Sep 3, 2009 #2
    You have a function that correlates vapor pressure with temperature, yes?

    What is the pressure at the normal boiling point?
     
  4. Sep 4, 2009 #3
    So if the normal boiling point is where the vapor pressure equals the external pressure, I can just set p2/p1 to 1.

    So I have ln1=-25.179/8.314(1/210-1/T) then T equals the normal boiling point? Eh, I'm confused.
     
  5. Sep 4, 2009 #4
    No.

    What is the difference between the definitions of "boiling point" and "normal boiling point"?
     
  6. Sep 4, 2009 #5
    Oh I get it now! The normal point is at STP. Ugh why didn't I see that before

    ln(760/65.3)=-25719/8.314(1/x-1/200)=~239K

    Thank you!!!
     
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