alleryStart learning →At a fixed temperature, how do lighter gas molecules compare with heavier ones?
AThey have a higher root mean square speed
BThey have a lower average kinetic energy
CThey have a lower root mean square speed
DThey have a higher average kinetic energy
Answer & Solution
Correct answer: A. They have a higher root mean square speed
1. Every gas at the same temperature carries the same average kinetic energy per mole.
2. Since kinetic energy depends on mass and speed together, a smaller mass must be balanced by a larger speed.
3. Lighter gases therefore have more high-speed particles and a higher root mean square speed.
4. Their speed distribution peaks further to the right, while heavier gases peak at lower speeds.
5. Average kinetic energy is set by temperature alone, so the two options about kinetic energy are wrong whichever way they point.
_Source: OpenStax Chemistry (CC BY 4.0), Ch 9 "Gases", section 9.5 The Kinetic-Molecular Theory_
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