What experiments can show the effect of molecular mass on diffusion?

Experiments involving the diffusion of different gases or the diffusion of dye in water can show the effect of molecular mass on diffusion.

One common experiment involves the diffusion of gases, such as hydrogen and carbon dioxide. This experiment is typically conducted in a long glass tube, with cotton wool soaked in hydrochloric acid at one end and ammonia at the other. Hydrogen, being lighter, diffuses faster and meets the hydrochloric acid, forming a white ring of ammonium chloride. This demonstrates that lighter molecules diffuse faster than heavier ones.

Another experiment involves the diffusion of dye in water. A drop of dye, such as food colouring, is added to a beaker of water. Over time, the dye will spread throughout the water. If you repeat this experiment with different dyes that have different molecular masses, you will observe that dyes with smaller molecular masses spread faster. This is because smaller molecules move faster and collide more frequently, leading to quicker diffusion.

A similar experiment can be conducted with perfume or a strong-smelling substance in a room. The time it takes for the smell to reach different parts of the room can be measured. Perfumes with lighter molecules will reach further parts of the room faster than those with heavier molecules.

These experiments all demonstrate the principle that the rate of diffusion is inversely proportional to the square root of the molecular mass. This principle, known as Graham's law of diffusion, is a fundamental concept in chemistry. It's important to remember that temperature and pressure can also affect the rate of diffusion, so these factors should be controlled in the experiments.