How does molecular mass affect diffusion speed in gases?

How does molecular mass affect diffusion speed in gases?

• A. Molecular mass does not affect diffusion.
• B. Lighter molecules diffuse slower.
• C. Diffusion speed increases with pressure.
• D. Lighter molecules diffuse faster than heavier ones.

Answer: (D)

In gases, how fast diffusion happens is tied to how fast the individual molecules move. At the same temperature, lighter molecules have higher average speeds than heavier ones, because their mass is smaller. Since diffusion relies on molecules moving and spreading into new areas, lighter gases diffuse into surrounding gases more quickly. This relationship is summed up by Graham’s law: the diffusion rate is inversely proportional to the square root of the molar mass. So lighter molecules diffuse faster than heavier ones. The idea that diffusion is unaffected by mass isn’t correct for gases under ordinary conditions, and diffusion speed isn’t primarily set by pressure at a fixed temperature (though pressure changes can affect concentrations, not the fundamental speed determined by molecular mass).

In gases, how fast diffusion happens is tied to how fast the individual molecules move. At the same temperature, lighter molecules have higher average speeds than heavier ones, because their mass is smaller. Since diffusion relies on molecules moving and spreading into new areas, lighter gases diffuse into surrounding gases more quickly. This relationship is summed up by Graham’s law: the diffusion rate is inversely proportional to the square root of the molar mass. So lighter molecules diffuse faster than heavier ones. The idea that diffusion is unaffected by mass isn’t correct for gases under ordinary conditions, and diffusion speed isn’t primarily set by pressure at a fixed temperature (though pressure changes can affect concentrations, not the fundamental speed determined by molecular mass).