[Concept: Avogadro's Law — equal volumes of gases at STP contain equal number of molecules. V ∝ n at constant T, P]

Numbered Labels

1. Equal volumes at same T, P — Both cylinders represent 1 L of gas; they contain the same number of molecules regardless of identity (Avogadro's Law).
2. Molecule count = Nₐ per mole — If each cylinder contains 1 mol of gas, it holds $6.022 \times 10^{23}$ molecules.
3. Volume = 22.4 L at STP — At 0 °C and 1 atm, 1 mol of any ideal gas occupies 22.4 L.
4. Different molecular masses — $H_{2}$ (M = 2 g/mol) and $O_{2}$ (M = 32 g/mol) contain the same number of molecules per mole, but very different masses.
5. Mass ≠ Volume — Heavier molecules make the same number of moles weigh more; volume at STP remains 22.4 L.
6. Bridge to laboratory — Avogadro's Law is what makes the mole concept experimentally useful: you can measure volume of gas to count moles.