1 Avogadro's hypothesis

• Solids and liquids of one mole have different volumes, because their particles are packed close together and differ in size.
• Gas particles are very far apart compared to their own size, so the volume of a gas depends almost only on the pressure and temperature, not on what the gas is.
• This is the content of Avogadro's hypothesis:

Equal volumes of different gases, under the same conditions of temperature and pressure, contain the same number of molecules.

Consequently, one mole of any gas occupies the same volume under the same conditions. Measurements showed:

• At STP (standard temperature and pressure: 0 °C = 273 K and 1 atm = 760 mmHg), one mole of any gas occupies 22.4 L.
• At rtp (room temperature and pressure: 25 °C = 298 K and 1 atm), one mole of any gas occupies 24 L.

The two values differ because a gas expands when warmed: at the higher temperature of rtp the same number of molecules needs more space.