When a gas is collected over water, it is not pure because it contains water molecules. The partial pressure of the collected gas can be determined by using Dalton's Law of partial pressures.
Work Step by Step
After the total pressure is determined, the pressure of the water vapor is determined by looking through a reference table (such as Table 5.4) at that specific temperature. The partial pressure of the collected gas in question can now be determined by subtracting the water vapor pressure from the total pressure.