Why would the reaction occur closer?

Question

Two cotton plugs are placed simultaneously in each end of a glass tube, 12 cm in length. On one end the cotton is soaked with aqueous NH3 and on the other end the cotton is soaked with aqueous HCl. Within minutes a white ring of NH4Cl appears about 4 cm from the HCl side. NH4Cl has formed as the HCl reacted with the NH3. Explain why the reaction occurred much closer to the HCl than to the NH3 side.

Answer 1

Diffusion of NH3 and HCl occurs and the rate of diffusion is dependent on the molar mass.  The lower the molar mass, the faster it will diffuse.  Molar mass of NH3 is 17 and that for HCl is 36.5, so NH3 diffuses faster, so at any given time, it will be closer to the HCl side than the HCl is to the NH3 side.