Question

at the same temperature, what is true of lighter, aka less massive, molecules?
lighter molecules are more impacted by intermolecular forces.
lighter molecules move slower.
lighter molecules move faster.
lighter molecules are less impacted by intermolecular forces.

Explanation:

According to the kinetic - theory of gases, at a given temperature, the average kinetic energy of all gas molecules is the same. Kinetic energy is given by $KE=\frac{1}{2}mv^{2}$. Since lighter molecules have a smaller mass $m$, to have the same kinetic energy as heavier molecules, they must have a higher velocity $v$. Intermolecular forces are not directly related to molecular mass in terms of being more or less impacted just based on mass at a fixed temperature.

Answer:

C. Lighter molecules move faster.