Calculating Heat of Vaporization for Substance X using Vapor Pressure Data

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SUMMARY

The discussion focuses on calculating the heat of vaporization for substance X using vapor pressure data at different temperatures. The correct formula applied is ln(P1/P2) = -ΔHvap/(R)(1/T2 - 1/T1), where ΔHvap is the heat of vaporization, R is the gas constant (8.314 J/mol·K), and T is the temperature in Kelvin. The participant consistently arrives at an answer of 41.3 kJ/mol, which is close to the expected value of 46.7 kJ/mol, indicating potential issues with experimental data accuracy or unit conversions. The importance of ensuring proper units for pressure and temperature is emphasized.

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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
 
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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.
 
cep said:
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.
 
Last edited:

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