By Riti Gupta
Updated Aug 30, 2022
Dizzy/iStock/GettyImages
When working in chemistry, the distinction between molecular weight and molar mass often trips up students and professionals alike. The difference is purely one of scale and units, but it is essential for accurate calculations in both the laboratory and industry.
A molecule is the smallest particle of a substance that still retains its chemical identity. For instance, a single molecule of carbon dioxide (CO₂) contains one carbon atom and two oxygen atoms; break it down further and it is no longer CO₂.
A mole (mol) is a unit that represents a fixed number of entities—Avogadro’s number, 6.022 × 10²³. It’s a convenient way to count atoms, molecules, or any other discrete particles. Thus, one mole of CO₂ contains 6.022 × 10²³ CO₂ molecules.
Molecular weight is the mass of a single molecule expressed in atomic mass units (amu). It is calculated by summing the atomic weights of all atoms in the molecule.
Example: Water (H₂O)
• Hydrogen: 1.008 amu
• Oxygen: 16.00 amu
\(\text{Molecular weight} = 2(1.008\,\text{amu}) + 16.00\,\text{amu} = 18.02\,\text{amu}\)
Molar mass is the mass of one mole of a substance and is expressed in grams per mole (g/mol). It is numerically equal to the molecular weight but uses a different unit to reflect the larger scale.
Example: Water (H₂O)
• Hydrogen: 1.008 g/mol
• Oxygen: 16.00 g/mol
\(\text{Molar mass} = 2(1.008\,\text{g/mol}) + 16.00\,\text{g/mol} = 18.02\,\text{g/mol}\)
Molecular weight = mass of one molecule (amu). Molar mass = mass of one mole of that substance (g/mol). The numeric values are identical; the units differ.
