How did Cannizzaro use Avogadro’s ideas to calculate atomic weights?

In the mid-19th century, chemical notation was in a state of chaos due to conflicting systems for determining atomic weights and molecular formulas. Stanislao Cannizzaro resolved this by reviving Amedeo Avogadro’s 1811 hypothesis, which stated that equal volumes of gases at the same temperature and pressure contain an equal number of molecules.213

Cannizzaro used Avogadro’s ideas to construct a consistent theoretical structure by:

• Vapour Density Analysis: He utilized vapour densities to infer the relative weights of atoms and molecules. By applying Avogadro’s hypothesis, he could determine the molecular weights of gases relative to a standard, such as hydrogen.13
• Distinguishing Atoms and Molecules: He clarified that elementary gases often consist of molecules containing more than one atom (e.g., diatomic molecules), which explained previous anomalies in chemical reactions.1
• Standardizing Formulas: By consistently applying these principles, he established a single, robust system of atomic formulas that aligned with empirical evidence, effectively replacing the competing, incompatible systems of the time.1

His work, presented in an 1858 pamphlet and later at the 1860 Karlsruhe Congress, provided the foundation for the modern system of atomic weights used today.14