Question

give reasons: a gas exerts pressure on the walls of the container

Answer 1

due to the random motion of molecules in side the vessel because they have least force of attraction and maximum kinetic energy.
it is for the same reason that when Air is blown inside balloon it inflates.

Answer 2

this to due to molecule's random motion in random directions and so crashes with walls of cotainer that's why they have energy and exerts pressure on walls of container too.
hope it helps u..☺