Collection of a Gas Over Water

Collection of a gas over water problems can be found in a variety of circumstances, but in each case a gas is collected by bubbling it into a container that is filled with water.

As it is collected, the gas displaces the water and is trapped in the container.

Generally the container holding the gas is marked to allow the measurement of volume, but getting the pressure is trickier.

The most direct way to find the pressure is to hold the collection container so that the level of water inside and outside is the same. In that circumstance, we know that the pressure inside the container and outside must be equal. (If you aren't sure why, go back and look at the material on straws again.)

There is, however, a problem. Water evaporates. That means that the gas inside the container is not pure. It is a mixture of the gas you are collecting and water vapor. So, while the pressure inside is still equal to the pressure of the atmosphere, the pressure inside is now the sum of two pressures:

P outside = P inside

Given that the outside pressure is due to the atmosphere, we can now write

P atmosphere = P gas + P water vapor

We can solve for the pressure of the gas by rearranging algebraically.

P gas = P atmosphere - P water vapor

Fortunately for us, the vapor pressure of water is temperature dependent. That means we can look up, on a table, the value of the water vapor pressure at our temperature. This, then, allows us to determine the pressure of the gas.

Note: there are two phrases which ALWAYS indicate this type of problem. The first is “collected over water,” and the second is “collected by water displacement.” Any time you see either of these phrases you will need to deal with the vapor pressure of water.

Here is a sample problem of this type: