Chapter 40, Problem 9A

Why do heavier molecules in a gas move more slowly on average than lighter ones at the same temperature?

To determine

The reason for slower speed of heavier molecules than lighter molecules of gas at same temperature.

Explanation of Solution

Introduction:

The energy of the particles in the motion is called kinetic energy. It depends on the mass and speed of the particle. If the speeds of two particles are same then lighter particle moves faster than heavier.

The kinetic energy of the gas molecules depends on the temperature of the molecules. If the temperatures of two molecules are same, then the kinetic energy of both molecules also become same.

The temperature of the heavy molecules and light molecules are same. So, the kinetic energy of both are also same. The mass of the heavier molecules is more than the lighter molecules. So, the speed of the lighter molecule is more than the heavier molecules and heavier molecules move slowly.

Conclusion:

Thus, the heavier molecules in a gas move slowly than the lighter molecules because the mass of the heavier molecules is more than the lighter molecules for same kinetic energy and same temperature.