# How do I calculate vapor pressure of water?

Dec 20, 2014

You could use Antoine's equation to calculate the vapor pressure of water. Antoine's equation is a vapor pressure equation that describes the relationship between vapor pressure and temperature for pure components. So,

${\log}_{10} p = A - \frac{B}{C + T}$, where

$p$ - the vapor pressure;
$T$ - temperature;
$A$, $B$, $C$ - constants specific to each substance.

You'd get $p = {10}^{A - \frac{B}{C + T}}$.

This equation is usually used with two sets of paramaters, a low-pressure paramater that describes the vapor pressure curve up to the normal boiling point, and a second paramater that covers the range from boiling point to critical point.

So, for water, the two sets of paramaters are

SET 1. $A$ = 8.07131, $B$ = 1730.63, $C$ = 233.426 for temeperatures between $1$ and ${100}^{\circ} C$;
SET 2. $A$ = 8.14019, $B$ = 1810.94, $C$ = 244.485 for temperatures betwen $99$ and ${374}^{\circ} C$.

Notice that you can use both sets of paramaters for temeperatures between $99$ and ${100}^{\circ} C$.

Here's a diagram of what an Antoine curve would look like