Molecular mass

Definition

Molecular mass is the mass of a single molecule, typically measured in atomic mass units (amu). It represents the sum of the masses of all the atoms present in that molecule, taking into account the number of each type of atom and their respective atomic weights. This concept is crucial for understanding various transport properties, as molecular mass affects how substances move through different media, including their rates of diffusion, viscosity, and thermal conductivity.

5 Must Know Facts For Your Next Test

1. Molecular mass is usually expressed in grams per mole (g/mol) when discussing larger quantities of molecules, while atomic mass units (amu) are used for individual molecules.
2. The higher the molecular mass of a substance, the slower its diffusion rate tends to be, as heavier molecules are less mobile.
3. In fluids, higher molecular mass typically leads to increased viscosity, meaning thicker fluids flow more slowly compared to lighter fluids.
4. Molecular mass also influences thermal conductivity; lighter molecules can transfer energy more efficiently than heavier ones.
5. Calculating molecular mass accurately is essential for predicting the behavior of substances during physical processes such as mixing and phase changes.

Review Questions

• How does molecular mass influence the rate of diffusion in different substances?
  • Molecular mass significantly impacts the rate of diffusion, as lighter molecules tend to move more quickly than heavier ones. This means that substances with lower molecular masses will diffuse faster through a medium compared to those with higher molecular masses. For example, gases with lower molecular weights will escape from a container more rapidly than those with higher weights due to their increased kinetic energy and mobility.
• Discuss how molecular mass affects viscosity and provide an example to illustrate this relationship.
  • Viscosity is influenced by molecular mass because larger molecules create more resistance to flow. For instance, honey has a higher viscosity than water due to its larger and heavier sugar molecules, making it flow more slowly. This relationship demonstrates that as molecular mass increases, the overall flow characteristics of a liquid change, leading to thicker or more viscous behavior.
• Evaluate the implications of molecular mass on thermal conductivity and its importance in practical applications.
  • Molecular mass plays a crucial role in determining thermal conductivity because it affects how efficiently energy is transferred between molecules. Lighter molecules typically have higher thermal conductivity because they can transfer kinetic energy rapidly during collisions. In practical applications, this understanding is vital when designing materials for insulation or heat exchangers, where optimizing thermal conductivity can lead to enhanced energy efficiency and performance.

Related terms

Diffusion:

The process by which molecules spread from areas of high concentration to areas of low concentration, influenced by molecular mass.

Viscosity:

A measure of a fluid's resistance to flow, which can be affected by the molecular mass of its components.

Thermal Conductivity: The ability of a material to conduct heat, which can also be influenced by the molecular mass of the molecules involved.