The solid will be formed at the point C owing to the fact that the HBr is heavier and will diffuse more slowly
Does molecular mass affect rate of diffusion?
Diffusion rate is affected by molecular mass. Graham's law of diffusion states that a gas's rate of diffusion is inversely related to its molar mass squared.
This means that, given the same temperature and pressure, lighter molecules will typically diffuse more quickly than heavier ones. The kinetic theory of gases, which states that lighter molecules diffuse at different rates because they move more quickly on average than heavier molecules, is what causes the relationship.