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      Question

      For two different gases at the same temperature, the gas

      with the lighter molecules (lower molecular mass) will have, on average:
      A Lower molecular speeds Correct Answer Incorrect Answer
      B The same molecular speed Correct Answer Incorrect Answer
      C Higher molecular kinetic energy Correct Answer Incorrect Answer
      D Higher molecular speeds Correct Answer Incorrect Answer

      Solution

      In kinetic theory of gases, the average kinetic energy of a gas molecule depends only on temperature and is independent of the type of gas. The formula for average kinetic energy per molecule is: where:

      • m = mass of one molecule
      • M = molar mass
      From this, we can see:
      • Smaller the mass (lighter molecule) ➔ higher the rms speed.
      • Heavier molecule ➔ lower the rms speed.
      Thus, for a lighter gas at same temperature:
      • It has the same average kinetic energy as heavier gas.
      • But it has higher average speed.

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