A box of ideal gas has consists of light particles and heavy particles (the heavy ones have 16 times the mass of the light ones). Initially all the particles have the same speed. When equilibrium is reached what will be true?

1. All the particles will still have the same speed
2. The average speed of the heavy particles will be equal to the average speed of the light particles
3. The average speed of the heavy particles will be larger than the average speed of the light particles
4. The average speed of the heavy particles will be smaller than the average speed of the light particles

Coming to equilibrium means coming to a particular temperature, which means the average kinetic energy of the particles is a particular value. The average kinetic energy of the light particles equals the average kinetic energy of the heavy particles - this can only happen if the average speed of the heavy particles is smaller than that of the light particles.

Smaller by a factor of what?

4.