Question

derive the expression of pressure exerted by the gas in the walls of the container

Answer 1

p=hdg, where
h=height, d=density.

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Answer 2

Because in the gaseous state, the particles move about randomly at high speed. Due to this random movement, the particles hit each other and also the walls of the container. The pressure exerted by the gas is because of this force exerted by gas particles per unit area on the walls of the container.