Mass

Mass is a fundamental property defined as the quantity of matter in an object or particle at rest relative to the observer, measured in kilograms ($\text{kg}$) or grams ($\text{g}$) ¹. Mass also quantifies inertia—the resistance of an object to changes in its state of motion ¹.

At the microscopic level, particles possess characteristic masses, quantified in chemistry as relative atomic mass ($A_r$) and relative molecular mass ($M_r$) ².

Relationship Between Mass and Diffusion

In Kinetic Particle Theory, at a given temperature, all particles share the same average kinetic energy ($\overline{E_k} = \frac{1}{2} m v^2$) ²:

• Heavier particles ($m \uparrow$) travel at lower average speeds ($v \downarrow$).
• Lighter particles ($m \downarrow$) travel at higher average speeds ($v \uparrow$).

Consequently, the rate of diffusion is inversely related to particle mass: substances with lower molecular mass diffuse faster than substances with higher molecular mass ².

Classical Demonstration: $\text{NH}_3$ vs. $\text{HCl}$ Diffusion

[Cotton wool + NH3(aq)]                             [Cotton wool + HCl(aq)]
   (Mr = 17, lighter)                                 (Mr = 36.5, heavier)
+-----------------------+-------------------------------------------------+
| =====>                |       White Ring of NH4Cl(s)             <===== |
| Faster diffusion      |       forms closer to HCl                       |
+-----------------------+-------------------------------------------------+
|<---- Long distance --><------- Short distance ------------------------->|

1. Ammonia gas ($\text{NH}_3, M_r = 17$) has smaller mass, traveling faster and further along the tube.
2. Hydrogen chloride gas ($\text{HCl}, M_r = 36.5$) has larger mass, traveling slower.
3. The solid white ring of ammonium chloride ($\text{NH}_4\text{Cl}$) forms distinctly closer to the $\text{HCl}$ end ².

Related

• Diffusion - Net transport of particles driven by concentration gradients.
• Particle - Discrete unit of matter possessing mass.
• Molecule - Chemical unit whose mass ($M_r$) dictates diffusion speed.
• Kinetic Particle Theory - Relates particle mass and speed to thermal energy.
• Chemistry - Core physical science area.