Question

gas molecules have speeds comparable with those of rifle bullets, yet it is observed that a gas with a strong odor (ammonia, for instance) takes a few minutes to diffuse through a room why?

Answer 1

A rifle bullet takes a straight path from the barrel to whatever it hits. A gas molecule changes direction many, many times.

If you do the math, you will find that an object that travels in a straight line goes double the distance in double the time (obviously enough). But an object that changes direction needs four times as long to go double the distance. (In general, to go N times as far, it needs N^2 as long.)

So, let's say a gas molecule moves at 100 feet per second. But changes direction (due to bumping into another molecule) on average every inch. (This is called the 'mean free path'.)

So it travels its first inch at 100 feet per second, or in about a thousandth of a second. To go fifty feet, or 600 times as far, it would need 360,000 times as long, or 360 seconds, or 6 minutes.

So the answer is that it's because of the mean free path of the gas molecules and the mathematics of traveling in random directions. (Sometimes in the right direction, sometimes back the way you came.)