# Constant C in Vapor Pressure Equation

1. Sep 29, 2009

### pzona

I'm working on vapor pressure for my general chemistry class, and I recently came across the curve P=C^(-$$\Delta$$H/RT). I was wondering what the units are for C (or are there no units because it is a constant?) and where I could find a table of values for common liquids (water, ethanol, methanol, diethyl ether, etc.). I know that I can cancel out C when actually doing problems, but I was curious to see the magnitudes of the values and how much they differ from each other. I haven't been able to find a table of values so far, so if anyone has a link I'd appreciate it.

2. Sep 29, 2009

### Mapes

The complete expression is

$$P=P_0\exp\left(\frac{\Delta S}{R}-\frac{\Delta H}{RT}\right)=P_0\exp\left(\frac{\Delta S}{R}\right)\exp\left(\frac{-\Delta H}{RT}\right)$$

where $P_0$ is the unit pressure (e.g., 1 Pa, 1 bar, 1 atm, etc.) and $\Delta S$ is the entropy increase when the condensed phase evaporates. This equation is derived by setting equal the chemical potentials of the condensed and vapor phases.