Free Kinetic Molecular Theory MCQs with Answers

1. Which assumption is made by the kinetic molecular theory of an ideal gas?

• A. Gas molecules attract one another strongly
• B. The volume of the molecules themselves is negligible compared with the volume of the container
• C. Collisions between molecules lose energy as heat
• D. The molecules all move at the same speed

Explanation: The ideal model treats molecules as point masses with negligible volume and no intermolecular forces, colliding perfectly elastically so no kinetic energy is lost. Molecular speeds are not uniform but spread over the Maxwell-Boltzmann distribution, with only the average kinetic energy fixed by temperature. Real gases deviate exactly where these assumptions fail.

2. The average kinetic energy of the molecules of an ideal gas depends only on

• A. pressure
• B. absolute temperature
• C. volume
• D. molar mass

Explanation: Average kinetic energy equals three halves of kT, so it is fixed by absolute temperature alone. This is why hydrogen and oxygen at the same temperature have the same average kinetic energy even though hydrogen molecules move much faster, being lighter. Molar mass affects speed, not energy.

3. Which assumption is NOT part of the kinetic molecular theory of an ideal gas?

• A. Gas molecules are in constant random motion
• B. Collisions between molecules are perfectly elastic
• C. There are strong attractive forces between gas molecules
• D. The volume of the molecules is negligible compared with the volume of the container

Explanation: An ideal gas is defined by the absence of intermolecular forces, which is precisely why it obeys the gas laws exactly. Real gases do have such forces, which is why they deviate at high pressure and low temperature. The other three assumptions are all part of the model.

4. The pressure exerted by a gas on the walls of its container is caused by

• A. the weight of the gas molecules
• B. collisions of the molecules with the walls
• C. attraction between molecules and the walls
• D. the chemical reaction of the gas with the walls

Explanation: Each collision delivers a small impulse, and the enormous number of collisions per second averages out into a steady pressure. Raising the temperature makes the molecules strike harder and more often, which is why pressure rises with temperature at constant volume. Weight contributes negligibly at ordinary scales.

5. According to Graham's law, the rate of diffusion of a gas is

• A. directly proportional to its molar mass
• B. inversely proportional to the square root of its molar mass
• C. independent of its molar mass
• D. proportional to the square of its molar mass

Explanation: Lighter molecules move faster at a given temperature, so hydrogen diffuses four times as fast as oxygen, whose molar mass is sixteen times greater and whose square root is four. This is the principle behind separating uranium isotopes by gaseous diffusion. The relationship holds for effusion through a small hole as well.

6. At constant volume, the pressure of a fixed mass of gas is directly proportional to its absolute temperature. This is

• A. Boyle's law
• B. Charles's law
• C. Gay-Lussac's law
• D. Avogadro's law

Explanation: Gay-Lussac's law covers the pressure temperature relationship at constant volume, and it explains why a sealed aerosol can explode when heated. Boyle's law fixes temperature and relates pressure to volume, while Charles's law fixes pressure and relates volume to temperature. Each law holds one variable constant, which is the quickest way to tell them apart.

7. The average kinetic energy of the molecules of any ideal gas depends only on

• A. the identity of the gas
• B. the absolute temperature
• C. the pressure
• D. the volume of the container

Explanation: At the same temperature, hydrogen and oxygen molecules have the same average kinetic energy, though the lighter hydrogen molecules must move much faster to achieve it. This is the microscopic meaning of temperature. It also explains Graham's law, since equal energy with unequal mass means unequal speed.