Final answer:
Among N_2, O_2, CO_2, and H_2O molecules, H_2O molecules are the fastest on average because they have the lightest mass. This is supported by Graham's Law of diffusion.
Step-by-step explanation:
The speed of gas molecules is dependent upon their mass, with lighter molecules moving faster, on average, than heavier ones. This is because all gases, at a given temperature, have the same average kinetic energy. So, among N_2 (molecular nitrogen), O_2 (molecular oxygen), CO_2 (carbon dioxide), and H_2O (water vapor), H_2O molecules would be the fastest because they are the lightest. Graham's Law of diffusion also supports this idea as it states that the rate of diffusion of a gas is inversely proportional to the square root of its molar mass. Therefore, a lighter gas like H2O diffuses quicker than a heavier gas like CO2.
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