Question

Why should the barometric pressure be corrected for the vapor pressure of water?

Answer 1

In the collection of gas over water, the gas evolves along with the evaporation of water. To determine the pressure of the gas, the vapor pressure of the water needs to be subtracted. And for this, barometric correction is very important. Because the water will keep on evaporating until the pressure of water becomes equal to the atmospheric pressure.

Complete answer:
Barometric pressure is the pressure within the atmosphere of the earth. It is also known as atmospheric pressure. Here the pressure is measured using a barometer. It is calculated in terms of bars or atmospheres.
When we do experiments in the lab, there are times when the gas that is evolving in the reaction needs to be collected. But this becomes difficult as the gases have very small densities. So to collect the gas evolved a setup is done. This is known as the collection of gas over water by the water displacement method. In this setup, there is a container where the reaction is taking place and a container for collecting the gas. This container for gas collection is filled with water and inverted in a reservoir of water. The gas that is evolved during the reaction is collected by attaching one end of the hose to the reaction container and inserting the other end into the inverted gas collection bottle. As the gas is created, the water gets displaced from the bottle.
The pressure inside the bottle is partially from the gas getting collected and partially from the water vapor that has escaped from the surface of the water in the jar. The water inside the jar will continue to evaporate until the pressure of water is equal to the vapor pressure at that particular temperature. To determine the pressure of the gas it is important to subtract the pressure due to water. This will give us the pressure of the rest of the gas. And so the barometric pressure should be corrected.

Note:
To calculate the pressure of the gas Dalton’s law is used. The formula is as follows: ${P_{gas}} = {P_{total}} - {P_{{H_2}O}}$, where ${P_{gas}}$ is the pressure of the desired gas, ${P_{{H_2}O}}$ is the vapor pressure of the water, and ${P_{total}}$ is the total pressure. After calculating the pressure of the gas, if the number of moles of gas needs to be calculated then the formula $PV = nRT$ can be used.