Final answer:
Increasing the molecular weight of a substance generally results in a slower rate of net diffusion due to the heavier molecules moving more slowly. Temperature and solvent density also affect diffusion speed, with higher temperatures speeding up and higher densities slowing down the diffusion process.
Step-by-step explanation:
If you increase the molecular weight of a substance, the effect on the rate of net diffusion is that the diffusion will generally be slower. This is because larger, heavier molecules move more slowly as compared to lighter ones, which is reflected in the diffusion constant D. The diffusion constant is influenced by several factors, including temperature, solvent density, and the mass of the molecules diffusing.
Higher temperature generally increases molecular movement, thereby increasing the rate of diffusion. Conversely, as the solvent density increases, which means the medium in which molecules are diffusing becomes denser, the rate of diffusion tends to decrease due to the molecules experiencing a more difficult time moving through the denser medium.
In summary, when the molecular weight increases, the molecules have less kinetic energy for movement at a given temperature, resulting in a decrease in the diffusion rate.