Q.Which of the following gases will have least rms speed at a given temperature?

Step 1. From Section 9.2.5, at a fixed temperature, $v_{rms}=\sqrt{3RT/M}\propto1/\sqrt M$ -- heavier molecules move slower on average.

Step 2. Compare molar masses: hydrogen $M\approx2\ \text{g mol}^{-1}$, nitrogen $M\approx28\ \text{g mol}^{-1}$, oxygen $M\approx32\ \text{g mol}^{-1}$, carbon dioxide $M\approx44\ \text{g mol}^{-1}$.

Step 3. Since carbon dioxide has the largest molar mass of the four gases listed, it must have the smallest $v_{rms}$ at any given common temperature. …

Rank the four gases by molar mass; the heaviest molar mass gi …

• Picking hydrogen, which actually has the HIGHEST rms speed since it is the lightest of the four gases. …