Important MCQs on Kinetic Theory with Explanation

Ans. (a) Graham’s law of diffusion

Explanation: Graham’s Law of diffusion states that at constant temperature and pressure, atoms or molecules with less molecular mass will effuse quicker than the ones with higher molecular mass atoms or molecules. In other words, it says that the rate of effusion of a gas is inversely proportional to the square root of the gas’s molecular mass. This formula is used for comparing the rates of two varying gases that are at equal temperatures and pressures.

Ans. (b) 1 mole of a gas

Explanation: The ideal gas equation can be written as

PV = nRT

Where

• V is the volume of the ideal gas.
• P refers to the pressure of the ideal gas
• n is the total ideal gas written in terms of moles.
• T is the temperature.
• R is the universal gas constant.

According to the Ideal Gas equation-

The product of Volume and Pressure of a gas has a constant relation with the Universal gas constant’s product, temperature, and the number of moles of gas, i.e. PV=nRT

Ans. (d) interact with each other through intermolecular forces

Explanation: An ideal gas is made of irregularly moving small molecules, which experience elastic collisions. Real gases are the ones that do not go according to the ideal relations of gas law. The deviation of real gas from ideal gas actions occurs due to the supposition that if pressure increases the volume decreases. The volume will become a lower number but won't be zero because the molecules will take some room that can not be compressed beyond. 

Ans. (c) in random motion

Explanation: The average kinetic energy per degree of freedom in a molecule’s translation motion must be equal to its rotational motion. So for a dynamic system in thermal equilibrium, which means that the molecules will be moving in various directions but still show a dynamic equilibrium in thermal energies, the system’s total energy is shared equally by all the freedom’s degrees. 

Therefore, we assign the term average kinetic energy to each degree of freedom with the help of the law, it is known that the constituents are going through the random motion. So the law of equal partition of energy is applied to the system where the constituents are moving randomly.

Ans. (b) ½ kT

Explanation: The degree of freedom is an independent physical parameter that specifies the state of a physical system. The degree of freedom dictates the number of ways in which a molecule in the gas phase may rotate, move, or vibrate in some space. The total number of degrees of freedom a molecule has plays a role in predicting the values of a lot of thermodynamic variables by using the equipartition theorem. 

For a system in equilibrium, there is an average energy of ½ RT or ½ kT per molecule related to each degree of freedom. (where T is the temperature of the system and k = Boltzmann constant). The energy related to each degree of freedom is in the form of potential and kinetic energy. 

Ans. (a) The molecules of a gas are always at rest

Explanation: The kinetic theory of gases describes the behaviour of ideal gases. Its five main postulates are as follows: 

• The particles of a gas are in random, constant motion, 
• The particles' combined volume is negligible, 
• No forces are exerted by the particles on one another, 
• Any collisions are completely elastic between the particles, and 
• The particles' average kinetic energy is directly proportional to the temperature (in kelvins).

Ans. (a) number of molecules.

Explanation: The law of Avogadro says that "equal volumes of all gases, at the same pressure and temperature, shall consist of the same number of molecules". For an ideal gas' given mass, the amount (moles) and volumes of the gas are directly proportional if the pressure and temperature are constant.

Ans. (a) all molecular linear velocities are zero.

Explanation: The zero temperature on a Kelvin scale is known as absolute zero. The absolute temperature is the same as 459.67 degrees Fahrenheit or minus 273-degree Celsius. All particles' movement stops and the disorder disappears at the absolute zero temperature. Therefore, there is nothing much colder than absolute zero.

Ans. (a) at low pressure and high-temperature

Explanation: Real gases approach ideal gas behaviour when they reach low pressure and high temperature because, at high pressure and low temperature, the molecules of gases get very close to each other. Molecular interactions start happening and the Repulsive forces also increase significantly. Repulsive interactions have a short-range interaction and become significant when molecules are almost very close in contact, whereas for an ideal gas the assumption is taken that there are no attractive forces between gas molecules. 

Real gas possesses ideal behaviour when conditions of pressure and temperature are such that the intermolecular forces are almost non-existent. This can only be obtained at very high temperatures and very low pressure.

Ans. (a) Perfect and of constant mass and temperature

Explanation: According to Boyle's law, the volume (V) of a given mass of a perfect gas at constant temperature is inversely proportional to its pressure (P). Mathematically, V ∝ (1/P) or PV = K.