Final answer:
Urea diffuses slower than Na+ because it is a larger molecule, which generally causes heavier molecules to diffuse more slowly.
Step-by-step explanation:
Urea diffused slower because it is larger than Na+. The rate of diffusion for a molecule is influenced by its mass—lighter molecules tend to diffuse more quickly than heavier molecules. In the given scenarios, the concentration gradient is also a critical factor in the diffusion process. For instance, when the concentration of solutes is higher in the dialysate relative to the blood, urea will diffuse to the lower dialysate because it moves from an area of higher concentration to lower concentration.
However, when comparing the diffusion rates of different molecules such as urea and Na+, one must also consider the size and mass of the molecules. Urea is a larger molecule than Na+ and therefore, will typically diffuse slower in a similar environment, assuming that both are diffusing passively without any carrier proteins or other mechanisms to facilitate their movement. This principle is also observed in other diffusion processes, such as the separation of heavy water (D₂O) from ordinary water (H₂O) due to the difference in their molar masses and thus their diffusion rates.