The key difference lies in the scale: molecular mass refers to the mass of a single molecule, while molar mass is the mass of one mole (6.022 x 1023) of those molecules.
Molecular Mass
- Definition: The sum of the average atomic masses of all atoms in a molecule. It represents the mass of a single molecule.
- Units: Atomic mass units (amu) or unified atomic mass units (u).
- Example: The molecular mass of water (H₂O) is approximately 18 amu (2 x 1 amu for hydrogen + 1 x 16 amu for oxygen).
Molar Mass
- Definition: The mass of one mole of a substance. A mole is a specific number of particles (Avogadro's number: 6.022 x 1023).
- Units: Grams per mole (g/mol). This is because a mole of a substance contains a massive number of molecules.
- Example: The molar mass of water (H₂O) is approximately 18 g/mol. This means that 6.022 x 1023 water molecules weigh 18 grams.
Practical Implications
Understanding the difference is crucial for stoichiometric calculations in chemistry. Molecular mass helps determine the relative masses of different molecules, while molar mass is essential for converting between mass and moles in chemical reactions, using the relationship:
- Moles = Mass (g) / Molar Mass (g/mol)
Summary Table
| Feature | Molecular Mass | Molar Mass |
|---|---|---|
| Definition | Mass of a single molecule | Mass of one mole (6.022 x 1023) of molecules |
| Units | amu or u | g/mol |
| Scale | Microscopic (single molecule) | Macroscopic (large number of molecules) |
The references support this distinction, emphasizing that molar mass relates to a mole of substance, while molecular mass focuses on a single molecule. The units (amu vs. g/mol) further highlight the difference in scale.
