Dalton's Law of Partial Pressure states that the total pressure in a container will be the sum of the individual pressures of the gases present in a container.
#P_t# = #P_1# + #P_2# + #P_3# + etc.
Here is a video which discusses an application of this law.
Many chem classes will perform an experiment to collect a gas (Hydrogen, butane etc.) in an upside-down eudiometer which is filled with water and placed into a container of water. As the gas enters the eudiometer, water is pushed out. The pressure of the gases in the eudiometer (collected gas + water vapor) will equal the atmospheric pressure in the room as long as the water level inside and outside of the eudiometer match. The pressure of the collected gas will be found by taking the room pressure and subtracting the vapor pressure of water.