How did Avogadro's hypothesis help correct problems in John Dalton's atomic theory?

John Dalton’s atomic theory, while foundational, contained a significant error regarding the nature of atoms and molecules. Dalton assumed that the simplest possible ratio for atomic combinations was always true, leading him to believe that molecules of an element were always single atoms and that compounds formed in the simplest one-to-one ratios (e.g., he proposed water was HO rather than $H_{2}O$).12

Amedeo Avogadro helped correct these issues by proposing two key hypotheses in 1811:
* Elemental gases can exist as molecules composed of multiple atoms rather than as separate, individual atoms.1
* Equal volumes of gases contain equal numbers of molecules.1

These insights reconciled Dalton’s theory with Joseph-Louis Gay-Lussac’s law of combining volumes. By recognizing that oxygen molecules consist of two atoms, for instance, scientists could explain why only half a volume of oxygen is required to react with a full volume of carbon monoxide to form carbon dioxide. This distinction between atoms and molecules eventually allowed for the development of accurate chemical notation and consistent atomic weights.12