# Vapor pressure and boiling point solution

This is a question about finding boiling point of water with the vapor pressure formula. The question states

In Breckenridge, Colorado, the typical atmospheric pressure is 520 torr. what is the boiling point of water (delta Hvap = 40.7 kJ/mol) in Breckenridge?

Using the vapor pressure formula

Do we pick the ln(P1/P2)=deltaHvap/R (1/T2-1/T1) or do we use the ln(P2/P1)= deltaHvap/R(1/T1-1/T2) and can you tell me why we pick it? I believe it is the second one but I am not sure why?

ln(52f0.torr/760.torr = 40.7 x 10 ^3 J/mol/ 8.3145 J/K mol ( 1/373K - 1/T2),

out of this I got -7.75 x 10 ^ -5 = 2.68 x 10^3 - 1/T2

How do I get 1/T2 = 2.76 x 10 ^ -3 from the last two numbers? I am not sure where 2.76 x 10 ^ -3 is coming from? I have tried to manipulate the numbers around, and cannot see how they are coming up with this number? From here getting the answer 89 degrees celcius, I understand.

Tthank-you very much.

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#### Solution Preview

The rule of logarithms states that ln(a/b) = -ln(b/a)

Therefore, you can use any of the two formulae, because they are equivalent.

The first formula states: ln(P1/P2) = ...