Question

Which of the following is incorrect statement for an ideal gas ?

• A. When temperature increases at constant pressure, collision frequency increases
• B. Mean free path decreases by increasing temperature at constant pressure
• C. If all gas molecules are assumed to be rigid sphere of negligible volume, the only possible molecular motion is translation
• D. Kinetic energy decreases by increasing pressure at constant temperature

Answer 1

Answer: (A), (B), (D)

The correct options are
A When temperature increases at constant pressure, collision frequency increases
B Mean free path decreases by increasing temperature at constant pressure
D Kinetic energy decreases by increasing pressure at constant temperature

(A) If the temperature increases at constant pressure the gas expands and the molecules of gases move far apart from each other i.e., the mean free path increases, thus the frequency of collision frequency decreases, so A option is wrong.

(B) For an ideal gas, if the temperature increases at constant pressure the gas expands and the molecules of gases move far apart from each other i.e., the mean free path increases. Hence, option B is wrong.

(C) If all gas molecules are assumed to be rigid sphere of negligible volume, the only possible molecular motion is translation. Hence, statement (C) is correct.
(D) For an ideal gas, the average kinetic energy is dependent only on temperature, for isothermal change in pressure, the average kinetic energy would not change. Hence, statement D is wrong.