How does the diffusion rate relate to molecular mass?

The diffusion rate is inversely proportional to the molecular mass, meaning as molecular mass increases, the diffusion rate decreases.

Diffusion is the process by which particles spread out from an area of high concentration to an area of low concentration. This process is fundamental in chemistry and is driven by the random motion of particles, often described as Brownian motion. The rate at which this happens, the diffusion rate, is influenced by several factors, one of which is the molecular mass of the particles involved.

Molecular mass refers to the mass of a single molecule, measured in atomic mass units (amu). It is calculated as the sum of the mass of all the atoms in a molecule. For example, the molecular mass of water (H2O) is approximately 18 amu, calculated by adding the masses of two hydrogen atoms and one oxygen atom.

The relationship between diffusion rate and molecular mass is an inverse one. This means that as the molecular mass of a substance increases, its diffusion rate decreases. Conversely, substances with lower molecular masses will diffuse more quickly. This is because heavier molecules move more slowly than lighter ones due to their greater inertia. Inertia is the resistance of any physical object to any change in its state of motion. Therefore, a molecule with a higher mass will have a greater inertia, and thus move more slowly, resulting in a lower diffusion rate.

This relationship is demonstrated in Graham's law of diffusion, which states that the rate of diffusion of a gas is inversely proportional to the square root of its molar mass. This law can be used to predict how quickly different gases will diffuse under the same conditions. For example, helium, which has a lower molecular mass, will diffuse faster than oxygen, which has a higher molecular mass.

In summary, the molecular mass of a substance plays a significant role in determining its diffusion rate. The greater the molecular mass, the slower the diffusion rate, and vice versa.