Molecular weight is the sum of the atomic weights of all the atoms in a molecule. It is usually expressed in atomic mass units (amu) or grams per mole (g/mol). This measurement helps scientists understand the mass of a substance and how it will behave in chemical reactions. Calculating molecular weight is essential in fields like chemistry and biology. For example, when working with compounds like water or glucose, knowing their molecular weights allows researchers to determine concentrations and predict how substances will interact in various conditions.