# Finding normal boiling point given pressure and enthalpy.

1. Sep 3, 2009

### webz

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. Sep 3, 2009

### PhaseShifter

You have a function that correlates vapor pressure with temperature, yes?

What is the pressure at the normal boiling point?

3. Sep 4, 2009

### webz

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. Sep 4, 2009

### PhaseShifter

No.

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

5. Sep 4, 2009

### webz

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!!!