Avogadro's Equal Volumes

Why it matters

Two balloons, same size, same temperature — one holds hydrogen, one holds oxygen. Believe it or not, they contain the same number of molecules.

Predict first

Equal volumes of two gases at the same temperature and pressure contain…

Avogadro's law: equal volumes of all gases, at the same temperature and pressure, contain equal numbers of molecules. So one mole of any gas fills the same volume (22.4 L at STP).

• the same number of molecules — correct
• the same mass
• the same number of atoms

What you do

Sort each statement: does Avogadro's law make it true, or not?

Check yourself

1 L of CO₂ and 1 L of H₂ at the same T and P contain…

Avogadro: equal volumes → equal molecule counts, whatever the mass.

Why does 1 mole of any gas fill 22.4 L at STP?

It follows from Avogadro's law — count sets volume, not size or mass.

• the same number of molecules — correct
• more molecules in the CO₂
• more molecules in the H₂
• Equal numbers of molecules take equal volumes at the same T, P — correct
• All gas molecules are the same size
• All gases weigh the same