Collecting a gas over water Steps of the process

How to do it 1. Any time we attempt to collect a gas via water displacement, we will finish the process with a mixture of gas and water vapor 2. The water vapor comes from the escape of water molecules from the surface of the liquid as they gain energy during the process, and accumulate in the mixture above the liquid 3. When the rate of escape of gas equals the rate of return, the number of water molecules become stable and the pressure of the water vapor remains constant 4. This pressure (which depends on temperature) is called the vapor pressure of water. This must be subtracted from the total pressure of the mixture and allows us to calculate the mass of the gas we collected via stoichiometry