Why did John Dalton use HO instead of H2O as the formula for water?
John Dalton used the formula HO for water because he lacked a method to determine the actual number of atoms in a molecule and relied on the assumption that nature follows the simplest possible rules of combination.12
At the time, Dalton believed that if two elements formed only one compound, that compound must consist of one atom of each element. Because there was no experimental way to determine molecular formulas, he assumed that one atom of hydrogen combined with one atom of oxygen to form a single molecule of water. This "simplest hypothesis" was a foundational, albeit mistaken, assumption in his atomic theory.21
It was not until later that the work of Amedeo Avogadro and the subsequent revival of his ideas by Stanislao Cannizzaro provided the evidence necessary to establish that water is composed of two hydrogen atoms and one oxygen atom, represented as HβO.12