Who reconciled Dalton's atomic hypothesis with Gay-Lussac's results on the combination of volumes in 1811?

Correct — D. In 1811 Avogadro proposed that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, and that elementary gases can be diatomic.

That one move fitted the results together. Gay-Lussac had gases combining in whole-number volume ratios, which Dalton's indivisible atoms could not explain: two volumes of hydrogen and one of oxygen give two volumes of water vapour.

If the oxygen particle is O₂, it divides between two water molecules with no atom cut. That is option (d).

Why the others are wrong

• (a)
  Robert Boyle — Robert Boyle gave the pressure-volume relation for a fixed mass of gas at constant temperature, in the seventeenth century — long before there was an atomic hypothesis to reconcile with anything.
• (b)
  Fred Hoyle — Fred Hoyle is a twentieth-century astronomer, known for stellar nucleosynthesis and the steady-state universe. The surname is there to catch a reader skimming for something like Boyle.
• (c)
  Jacques Charles — Jacques Charles gave the volume-temperature law. His work is about how a gas responds to heating, not about how many particles a given volume holds.

Avogadro's hypothesis works by separating two ideas that Dalton had run together: the atom, the smallest particle of an element, and the molecule, the smallest particle that exists on its own in a gas.

Once hydrogen and oxygen gas are allowed to be diatomic, Gay-Lussac's whole-number volume ratios follow directly from counting molecules.

The idea was largely ignored for about fifty years, until Cannizzaro used it to put atomic and molecular masses on a consistent footing.

Key facts

• Avogadro's hypothesis (1811): equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
• It reconciled Dalton's atomic theory with Gay-Lussac's law of combining volumes.
• It requires elementary gases such as hydrogen and oxygen to be diatomic.
• Avogadro's constant, 6.022 × 10²³ per mole, is named after him.

Common traps

• Reading the paper's 'Amadeo' as somebody other than Amedeo Avogadro.
• Choosing Boyle or Charles because both are gas-law names, though neither is about counting particles.

No directly related past PYQ was found.