Question

What role does water vapor pressure play in molar gas experiements?

Answer 1

the gas density is very less. it is not possible to collect the gases to determine their mass. Hence the gases are collected over water. This can be done for gases which are not soluble in water.

The experimental setup for the collection of a gas over water will have a container in which the reaction takes place and a gas collection container filled with water and inverted in a reservoir of water. The gas liberated from the reaction is collected by attaching one end of a hose to the reaction container and inserting the other up into the inverted gas collection bottle. As the gas is created, it will displace water from the bottle. The volume of gas can be determined by the amount of water that was displaced by the gas.

when the gas get collected , the water level in the container will adjust so that the pressure inside and outside the container are the same.

In that case, the total pressure = partial pressure of water vapor+ saturation pressure of water vapor

at saturation, partial pressure of water = vapor pressure of liquid

hence partail pressure of gas= total pressure- saturation vapor pressure of water.

the partial pressure of gas which is defined as the pressrue exerted if the gas alone occupies the entire volume, the partial pressure can be used to calculate the moles of gas using gas law equation.

PV= nRT

n= PV/RT