According to kinetic theory of gases, the pressure exerted by the gas on the wall is equal

1. Analyze the question The kinetic theory explains gas pressure in terms of molecules colliding with the walls of their container. We need the statement that correctly defines pressure in this picture.

2. Concept

• Pressure = Force / Area.
• By Newton's second law, Force = rate of change of momentum = Δp / Δt.
• So Pressure = (rate of change of momentum) / Area.

In the kinetic theory, each molecule hitting a wall rebounds, so its momentum changes. The wall receives an impulse from every collision. The very large number of collisions per second gives a steady force on the wall. That force divided by the wall area is the pressure.

3. Step-by-step solution

1. A molecule of mass m hits the wall with velocity u and rebounds with -u. Its change in momentum is 2mu.
2. Many molecules do this every second, so the total momentum transferred to the wall each second is large and fairly constant.
3. Force on wall = total momentum change per second.
4. Pressure = force / area = momentum change per second per unit area.

4. Checking the options

• A. "Per unit volume" is wrong, since pressure is related to area, not volume.
• B. "Rate of change of momentum ... per second per unit area" matches P = F/A with F = Δp/Δt. This is the closest correct statement. (Strictly, the momentum is imparted to the walls by the molecules, but the intended meaning is clear and the rest of the wording is right.)
• C. Momentum imparted per unit area has no time factor, so it is not a pressure.
• D. Change in momentum per unit area also has no time factor, so it is not a pressure.

Only B includes the "per second" (rate) idea, which is what turns momentum change into force.

Answer: B

Tip: Remember the chain Pressure → Force/Area → (Δp/Δt)/Area. If an option for pressure doesn't mention a rate (per second) and area, eliminate it.