# Finding normal boiling point given pressure and enthalpy.

webz

## 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

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

What is the pressure at the normal boiling point?

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.

PhaseShifter
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"?

webz
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!!!