## Conceptual Physics (12th Edition)

The species of gas molecules are mixed, which means they have the same temperature. This means they have the same average translational kinetic energy per molecule. $KE = \frac{1}{2} mv^{2}$. The lighter molecules (e.g., hydrogen) must have a higher average speed than heavier molecules (e.g., carbon dioxide) to have the same average kinetic energy per molecule.