Question

A closed vessel is maintained at a constant temperature. It is first evacuated and then vapour is injected into it continuously. The pressure of the vapour in the vessel is:

• A. increases continuously
• B. first increases and then remains constant
• C. first increases and then decreases
• D. none of the above

Answer 1

Answer: (B)

first increases and then remains constant

The molecules of a gas are in a state of random motion. They continuously collide against the walls of the container. Even at ordinary temperature and pressure, the number of molecular collisions with walls is very large. During each collision, certain momentum is transferred to the walls of the container. The pressure exerted by the gas is due to continuous bombardment of gas molecules against the walls of the container. Due to this continuous bombardment, the walls of the container experience a continuous force which is equal to the total momentum imparted to the walls per second. The average force experienced per unit area of the walls of the container determines the pressure exerted by the gas. This should be clear from the fact that although the molecule collisions are random, the pressure remains constant.