Calculating Heat of Vaporization for Substance X using Vapor Pressure Data

[priscilla98]

Homework Statement

Using the following vapor pressure data solve for the heat of vaporization for substance x.

T VP

20 100
30 175
40 325

Homework Equations

ln (P1/P2) = -Heat of vap/RT (1/T2 - 1/T1)

The Attempt at a Solution

I know the answer is 46.7 kJ/mol. But I'm getting a different answer.

ln (175/100) = x/(8.314) (1/303 - 1/298)
x = 41305 J
x = 41.3 kJ/mol

I'm always getting an answer close to 46.7 kJ/mol but not the exact answer.

Any help is greatly appreciated, thanks

 

[cep]

Does trying different combinations of the temperatures and pressures give the same (incorrect) answer? I noticed you didn't give a unit for pressure; make sure that you don't need to convert to a different unit, as you did with temperature.

 

[Borek]

I noticed you didn't give a unit for pressure; make sure that you don't need to convert to a different unit, as you did with temperature.

Doesn't matter, p1/p2 is unitless.

If these are experimental values they can be slightly off, perhaps that's what the exercise is about.