How does molecular weight affect the rate of diffusion?

1 Answer


At equal temperatures, lighter particles will diffuse more rapidly than heavier particles.


Diffusion is the movement of particles from where they are more concentrated to where they are less concentrated.

The average kinetic energy of a particle (KE) gives us the temperature of a substance.

KE = #1/2mv^2#

If we have two gases (A and B) with gas A being heavier than B. The kinetic energy of the two gases will be the same if the gases are at the same temperature.

#KE_A = KE_B#

So at equal temperatures, a lighter particle (B) must have a greater velocity (speed) and a heavier particle (A) must have a lower speed.

#1/2m_A(v_A)^2# = #1/2m_B(v_B)^2#

Here is a video which gives further explanation of this topic. It discusses effusion, which is basically the diffusion of particles through a small opening in a container.

Hope this helps!