What is the pressure?
You need to do a lab experiment where hydrogen gas is generated. In order to calculate the yield of gas, you have to know the pressure inside the tube where the gas is collected. But how can you get a barometer in there? Very simple: you don’t. All you need is the atmospheric pressure in the room. As the gas pushed out the water, it is pushing against the atmosphere, so the pressure inside is equal to the pressure outside.
Gas Collection by Water Displacement
Gases that are produced in laboratory experiments are often collected by a technique called water displacement (see Figure below). A bottle is filled with water and placed upside-down in a pan of water. The reaction flask is fitted with rubber tubing which is then fed under the bottle of water. As the gas is produced in the reaction flask, it exits through the rubber tubing and displaces the water in the bottle. When the bottle is full of the gas, it can be sealed with a lid.
Because the gas is collected over water, it is not pure but is mixed with vapor from the evaporation of the water. Dalton’s law can be used to calculate the amount of the desired gas by subtracting the contribution of the water vapor.
In order to solve a problem, it is necessary to know the vapor pressure of water at the temperature of the reaction (see Table below). The sample problem illustrates the use of Dalton’s law when a gas is collected over water.
Sample Problem: Gas Collected by Water Displacement
A certain experiment generates 2.58 L of hydrogen gas, which is collected over water. The temperature is 20°C and the atmospheric pressure is 98.60 kPa. Find the volume that the dry hydrogen would occupy at STP.
Step 1: List the known quantities and plan the problem.
VTotal=2.58 L T=20∘C=293 K PTotal=98.60 kPa=739.7 mmHg
VH2 at STP=? L
The atmospheric pressure is converted from kPa to mmHg in order to match units with the table. The sum of the pressures of the hydrogen and the water vapor is equal to the atmospheric pressure. The pressure of the hydrogen is found by subtraction. Then, the volume of the gas at STP can be calculated by using the combined gas law.
Step 2: Solve.
Step 3: Think about your result.
If the hydrogen gas were to be collected at STP and without the presence of the water vapor, its volume would be 2.28 L. This is less than the actual collected volume because some of that is water vapor. The conversion using STP is useful for stoichiometry purposes.
- The vapor pressure due to water in a sample can be corrected for in order to get the true value for the pressure of the gas.
Use the resource below to answer the questions that follow.
- What was the thistle tube used for?
- How did the instructor tests for oxygen?
- Did you observe any unsafe lab practices in the video?
- What would have happened to the splint if carbon dioxide had been collected?
- Why is gas collected over water not pure?
- Why would we want to correct for water vapor?
- A student wants to collect his gas over diethyl ether (vapor pressure of 530 mm Hg at 25°C). Is this a good idea? Explain your answer.