If you opened a container of each gas in a corner of a large room, which odor would you detect first on the other side of the room?

Gas molecules spread through the room by diffusion. Ammonia, NH₃, has a molar mass of about 17 g/mol, while hydrogen sulfide, H₂S, is about 34 g/mol. The lighter NH₃ molecules diffuse faster, so you would expect to detect the ammonia odor first.

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