# Using boiling temperature as a proxy for vapor pressure

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## Main Question or Discussion Point

So, we know that isopropanol is more volatile than water. We know that since isopropanol has a higher vapor pressure (40 mmHg at 23.8 degrees Celsius, whereas water is 17.5 mmHg at 20 degrees Celsius). We also know that isopropanol has a lower boiling point than water.

Now, are there molecules that have higher vapor pressures than water *and* higher boiling points than water? The chart at http://upload.wikimedia.org/wikipedia/commons/9/96/Vapor_Pressure_Chart.png shows very few intersections. But I wonder - are there cases where the vapor pressure curves for two molecules do intersect?

Also, how do we objectively compare the vapor pressures between two molecules? Do people try to measure the vapor pressure for a specific number of moles of a molecule? (and set the number of moles of N2 equal to the number of moles of H2O, if they want to compare the vapor pressures between the two molecules?)

Mapes
Homework Helper
Gold Member
Now, are there molecules that have higher vapor pressures than water *and* higher boiling points than water?
The boiling point is the temperature at which the vapor pressure equals 1 atm, so not simultaneously, no.

But I wonder - are there cases where the vapor pressure curves for two molecules do intersect?
The vapor pressure $p$ should scale as

$$p\propto \exp\left[-L\left(\frac{1}{T}-{\frac{1}{T_\mathrm{B}}\right)\right]$$

where $L$ is the latent heat and $T_\mathrm{B}$ is the boiling temperature. So it would seem that this could occur with two materials if one had the higher latent heat and the other had the higher boiling temperature.