Final answer:
The gas collection method described is water displacement, and it is used to collect gases like hydrogen in a reaction. The volume of water displaced equals the collected gas volume, but Dalton's Law is needed to separate the partial pressures of hydrogen and water vapor to determine the actual yield of the gas.
Step-by-step explanation:
Gas Collection by Water Displacement
The gas collection procedure described is a laboratory method known as water displacement. In this method, the hydrogen gas (H₂) produced in a reaction is directed through rubber tubing into an inverted graduated cylinder filled with water. As H₂ is produced, it displaces the water in the cylinder since it does not dissolve in water. The volume of water displaced is equivalent to the volume of the gas collected. However, this gas is not pure H₂; it is mixed with water vapor. To determine the actual amount of H₂ collected, one must apply Dalton's Law to account for the partial pressures of the gases—H₂ and water vapor.
The water's vapor pressure is a temperature-dependent value and represents the part of the total pressure that is due to water vapor. The pressure of the collected gas can be equalized with atmospheric pressure, negating the need for a barometer inside the collection container. Therefore, one can use the atmospheric pressure in the lab and the known vapor pressure of water to calculate the correct partial pressure for the hydrogen gas using Dalton's Law where PTotal = Pg+PH₂₀.