Molecular mass is expressed as the mass of a molecule in unified atomic mass units, while the number average molecular weight is expressed as the total weight of all molecules in a substance, divided by the overall number of molecules in a substance. Molecular mass is not referred to as "number average molecular mass."
The number average molecular weight is also known as the relative molecular mass and is numerically equivalent to the molecular mass of a molecule, with differing units. Due to their formulas, molecular weight and molecular mass are commonly mistaken as meaning the same thing. Molecular mass focuses on the isotopic composition of single molecules instead of the overall number distribution of the molecules in a substance and is used as a more precise form of determining the properties of molecules than measurements such as molar mass. Molar mass determines the average isotopic distribution in multiple molecules instead of single molecules and is commonly used to determine properties of bulk samples with accurate results.
Both molecular weight and molecular mass commonly deal with complex molecules, such as polymers, carbohydrates and proteins. These molecules require accurate measurements to determine their properties because of their complexity and size. Determining their molecular mass is more effective in understanding them than their molar mass, due to its focus on single molecules.