What does the molecular weight of 18 g for ordinary water signify?

The molecular weight of 18 grams for water (H2O) signifies the mass of one mole of water molecules. According to Avogadro's law, one mole of any substance contains Avogadro's number of entities, which is approximately (6.022 \times 10^{23}) molecules. Therefore, when referring to the molecular weight of water, it indicates that 18 grams of water corresponds to one mole, and thus contains Avogadro's number of water molecules.

This connection is crucial in stoichiometry and the study of chemical reactions, as it allows chemists to convert between the mass of a substance and the number of molecules or moles present. Understanding this relationship is foundational for accurate calculations in chemistry, ensuring that technicians can effectively measure and manipulate substances in various scientific applications.