I am trying to understand the variation in boiling point with pressure of substances.

Specifically, there is this graph ("nomograph") that is used to estimate the change in boiling point with pressure. This graph must be the representation of some equation, but which one? (I know this is an approximation, btw.)

At first, I thought it might be the "Clausius-Clapeyron" equation, P = P0 * exp( (-DH/R) * (1/T - 1/T0)).

I found something called "Trouton's Rule" that says the entropy change upon vaporization is about 21 cal/mol-K for many substances. Since DH = DS * T during vaporization, the above equation can be re-arranged to give:

T = T0 * (1 / (1 - 0.095 * ln(P/P0)))

where:

T = boiling point at reduced pressure, in K

T0 = boiling point at 1 atm, in K

P = reduced pressure

P0 = 1 atm

This doesn't seem to match the nomograph, however. As an example, CU Boulder has some example questions about reduced pressure:

http://orgchem.colorado.edu/hndbksupport/dist/distsq.htmlhttp://orgchem.colorado.edu/hndbksupport/dist/distsqans.htmlQuestion 8 in this is as follows:

Eugenol has a boiling point at 760 torr of 255 C. What is the boiling point at 25 torr (water aspirator) and at 0.1 torr (vacuum pump)?

Applying my equation, I get 125.6 C for the bp at 25 torr, and 12.6 C at 0.1 torr. The example, however, shows that the mysterious nomograph gives 145 C at 25 torr, and 55 C at 0.1 torr, quite different.

So, the nomograph represents a different equation than mine. Which one?

Thanks.