# How do you calculate the partial pressure of water?

May 19, 2016

Do you refer to the saturated vapour pressure?

#### Explanation:

The $\text{SVP}$ is a function of temperature, and is tabulated at various temperatures. Of course, at water's boiling point under standard conditions, the $\text{SVP}$ is $\text{1 atmosphere}$.

When you collect a gas by bubbling it thru water to a graduated cylinder, this gas is saturated with water vapour.

Thus ${P}_{\text{laboratory"=P_"gas"+P_"SVP}}$.

At $25$ $\text{^@C, "SVP"="23.8 mm Hg} .$ So you have to subtract this $\text{SVP}$ from the laboratory pressure in order to find ${P}_{\text{gas}}$, the pressure exerted by whatever gas you are collecting.

$\text{SVP}$ is commonly reported in $\text{mm Hg}$ because the mercury manometer is still the chemists' means of measuring pressure. Clearly, you don't need to memorize these values; they should be provided in an exam.