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What is the equilibrium vapor pressure of
a liquid? How is it measured?

The equilibrium vapour pressure of  a liquid is the pressure exerted by a vapour in a closed vapour and gas system which is in thermal equilibrium.

It is measured using a simple manometer

Explanation:

When liquid and vapour are in a closed container, the molecules of the liquid and vapour at a certain temperature, will attain thermal equilibrium. this means that there is no net transfer of heat between the bodies. The pressure exerted by the vapour on the liquid at this point is called the equilibrium vapour pressure.

To measure the equilibrium vapour pressure, the instrument used is a manometer. To use this, a sample of the liquid is dropped in a sealed flask connected to a manometer, and the increase in the pressure of the system is monitored as the fluid evaporates. This increase in pressure is known as the vapour pressure.