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 behaves in chemical reactions. Calculating molecular weight is essential in fields like chemistry and biology. For example, knowing the molecular weight of a compound like water (H₂O) allows researchers to determine how much of it is needed for reactions or to create solutions.