# How do you find the pressure of a gas in a eudiometer tube?

## I really just need someone to tell me the steps because I can't find them. Here's some more information, though: the gas is H_"2", the room temperature is 24.7C, and the barometric pressure is 766 torr. Thanks.

Jan 10, 2017

Here's how you can do that.

#### Explanation:

The idea here is that the gas is being collected over water, which basically means that the tube will contain hydrogen gas and water vapor.

A typical set up involving a eudiometer tube looks like this

Now, you know the temperature at which the gas is being collected, so you can look up the vapor pressure of water at that temperature. In this case, you have

P_( "H"_2"O") ~~ "23.56 torr"

http://www.endmemo.com/chem/vaporpressurewater.php

Now you can use Dalton's Law of Partial Pressures to figure out the partial pressure of hydrogen gas in the mixture.

color(blue)(ul(color(black)(P_ "total" = P_ ("H"_ 2) + P_ ("H"_ 2"O"))))

You will have

${P}_{\text{H"_ 2) = P_"total" - P_ ("H"_ 2"O}}$

Keep in mind that the total pressure is simply the barometric pressure, i.e. the pressure of the hydrogen gas + water vapor mixture.