Why were chemists uncertain about the formulas of compounds in the 1800s?

In the 1800s, chemists faced significant uncertainty regarding the formulas of compounds because they lacked a direct method to weigh individual atoms and molecules. Without this ability, they could not unambiguously determine the number of atoms present in a compound, leading to the development of several incompatible atomistic systems across different countries.12

Early pioneers like John Dalton relied on assumptions based on the "simplicity of nature," such as the belief that molecules of an element were single atoms. This led to incorrect formulas, such as designating water as HO rather than $H_{2}O$. Furthermore, the influential chemist Jöns Jacob Berzelius incorrectly contended that atoms of the same element repelled each other due to identical electrical charges, which blocked the acceptance of Amedeo Avogadro's hypothesis that gases could consist of polyatomic molecules.324

This state of chaos in chemical notation persisted until Stanislao Cannizzaro revived Avogadro’s ideas in 1858. By demonstrating that these theories consistently fit empirical evidence, Cannizzaro helped establish a unified, robust system of atomic weights and formulas at the 1860 Karlsruhe Congress.315