Water vapor pressure is an important consideration in gas volume calculations, as it must be subtracted from the total pressure of the gas mixture to obtain the pressure of the dry gas.

When gas is collected over water, it typically becomes saturated with water vapor, resulting in a mixture that includes both the gas of interest and water vapor. According to Dalton’s Law of Partial Pressures, the total pressure of this gas mixture is equal to the sum of the individual partial pressures of the gases present. This means that the total pressure can be expressed as:

where $P_{gas}$ is the partial pressure of the collected gas and $P_{H_2O}$ is the partial pressure of the water vapor.

To isolate the pressure of the dry gas, the vapor pressure of the water must be subtracted from the total pressure of the gas mixture. The vapor pressure of water at a given temperature can be obtained from steam tables.

For instance, consider a scenario where gas is collected over water at a temperature of $25^\circ C$. At this temperature, the water vapor pressure is approximately $23.8 \, \text{mmHg}$. If the total pressure of the gas mixture is $760 \, \text{mmHg}$ (which corresponds to standard atmospheric pressure), the pressure of the dry gas can be calculated as follows:

Once the pressure of the dry gas is determined, it can be utilized for gas volume calculations. The ideal gas law, represented by the equation:

provides the relationship between pressure ($P$), volume ($V$), number of moles ($n$), the gas constant ($R$), and temperature ($T$). It is crucial to ensure that the pressure used in these calculations reflects the pressure of the dry gas, rather than the total pressure of the gas mixture.