The movement of particles in a gas is inversely proportional to the square root of their molar mass. In other words, the heavier the molecule, the slower it moves. Hydrochloric acid (HCl) has a molar mass of 36.5 g/mol, while ammonia (NH3) has a molar mass of 17 g/mol. Therefore, HCl particles move faster than NH3 particles because they are lighter.

To put it simply, imagine two people, one carrying a heavy suitcase and the other carrying a light backpack. The person with the light backpack can walk faster than the person with the heavy suitcase because they have less weight to carry.