'Equal volumes of all gases, at the same temperature and pressure, contain the same number of molecules.' This is:

In 1811, the Italian scientist Amedeo Avogadro proposed that equal volumes of all gases, under the same conditions of temperature and pressure, contain an equal number of molecules. In other words, at constant temperature and pressure, the volume of a gas is directly proportional to the number of molecules (or moles) of the gas: V ∝ n.

Avogadro's hypothesis was important because it helped scientists distinguish between atoms and molecules. It explained Gay-Lussac's law of combining volumes, which showed that gases react in simple whole-number ratios by volume. For example, two volumes of hydrogen react with one volume of oxygen to give two volumes of water vapour. Avogadro suggested that hydrogen and oxygen exist as diatomic molecules (H2 and O2). His ideas were not widely accepted until 1860, when Stanislao Cannizzaro explained their importance at the Karlsruhe Congress.

Consequences of Avogadro's law:

One mole of any gas at standard temperature and pressure (STP, 0°C and 1 atmosphere) occupies about 22.4 litres. This is called the molar volume of a gas. (Under the newer IUPAC standard of 0°C and 1 bar, it is about 22.7 litres.)

The number of particles in one mole of any substance is called the Avogadro number (or Avogadro constant), about 6.022 × 10²³. It was named in his honour; it was first estimated by Josef Loschmidt in 1865. Since 2019, its value has been fixed exactly in the SI system.

Avogadro's law combines with Boyle's law and Charles's law to give the ideal gas equation, PV = nRT.

Other gas laws:

• Boyle's law: at constant temperature, pressure is inversely proportional to volume.
• Charles's law: at constant pressure, volume is directly proportional to absolute temperature.
• Graham's law: the rate of diffusion of a gas is inversely proportional to the square root of its density (or molar mass).
• Dalton's law: the total pressure of a mixture of non-reacting gases equals the sum of their partial pressures.

Why not the other options?

• ✗Boyle's law relates pressure and volume at constant temperature.
• ✗Charles's law relates volume and temperature at constant pressure.
• ✗Graham's law deals with the diffusion of gases.