Finding normal boiling point given pressure and enthalpy.

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  • #1
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Homework Statement


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



Homework Equations


ln(p2/p1)=-deltaH/R(1/t2-1/t1)


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.
 

Answers and Replies

  • #2
277
1
You have a function that correlates vapor pressure with temperature, yes?

What is the pressure at the normal boiling point?
 
  • #3
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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.
 
  • #4
277
1
So if the normal boiling point is where the vapor pressure equals the external pressure, I can just set p2/p1 to 1.

No.

What is the difference between the definitions of "boiling point" and "normal boiling point"?
 
  • #5
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No.

What is the difference between the definitions of "boiling point" and "normal boiling point"?

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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