What is the effect of molecular weight on diffusion rate?
The correct answer and explanation is:
Correct Answer:
As molecular weight increases, the diffusion rate decreases.
Explanation:
The rate of diffusion is influenced by several factors, and one of the most important is the molecular weight (or molar mass) of the diffusing substance. According to Graham’s Law of Diffusion, the rate of diffusion of a gas is inversely proportional to the square root of its molecular weight. This means that substances with lower molecular weight diffuse faster than those with higher molecular weight.
This relationship can be expressed mathematically as:
Rate of diffusion ∝ 1 / √M
Where M represents the molar mass of the substance.
The reason for this is kinetic. All particles at a given temperature have the same average kinetic energy. However, kinetic energy is related to both the mass and velocity of a molecule. Lighter molecules must move faster than heavier ones to have the same kinetic energy. Because lighter molecules move faster, they collide with other molecules and spread out more quickly, leading to a faster diffusion rate.
For example, consider two gases: hydrogen (H₂) and oxygen (O₂). Hydrogen has a much lower molecular weight (approximately 2 g/mol) compared to oxygen (about 32 g/mol). As a result, hydrogen will diffuse roughly four times faster than oxygen, since √(32/2) = √16 = 4.
In liquid or solid phases, diffusion also depends on molecular size, but the principle remains: larger molecules encounter more resistance and move more slowly through the medium.
This concept is important in many areas such as biology, chemistry, and medicine. For example, in respiratory physiology, gases like oxygen and carbon dioxide diffuse across cell membranes, and their rates of diffusion impact how effectively cells exchange gases with the environment.