Do equal volumes of gas hold equal numbers of particles?

That time, that place

After Dalton's atomic theory chemistry hit a wall. Electrolyse water and you get hydrogen and oxygen in a two-to-one ratio by volume — but one-to-eight by weight.

To get the weight of a single atom you must connect the two, and the connection would not come.

Why this question

Avogadro proposed that at the same temperature and pressure, equal volumes of any gas contain equal numbers of particles.

And he added one more thing: gases like hydrogen and oxygen do not float as single atoms but travel in pairs.

Put the two together and the numbers close. A ratio of volumes becomes a ratio of particle counts.

What was found

Chemistry ignored it for nearly fifty years. Identical atoms pairing up contradicted the bonding theory of the day, which held that atoms join only to unlike atoms.

At the 1860 Karlsruhe congress Cannizzaro rebuilt the table of atomic weights on Avogadro's hypothesis and showed that it worked. Only then was it accepted — four years after Avogadro's death.

Every calculation in chemistry now rests on it. The mole, and the number of particles in one, carry his name.