Loschmidt constant

The Loschmidt constant is Avogadro's number divided by the molar volume: how many molecules actually sit in a cubic metre of gas at 0 °C and 1 atm. At 2.69 × 10¹⁹ per cubic centimetre it explains, among other things, why the sky is blue — Rayleigh scattering depends directly on the number density of scatterers — and it sets the mean free path of about 68 nm that governs vacuum technology and gas viscosity.

Josef Loschmidt earned the name in 1865 by combining the kinetic theory of gases with the density of liquid air to estimate the size of an air molecule, and thence their number in a given volume. He came within a factor of a few of the modern value, the first credible measurement of the granularity of matter. At the IUPAC 100 kPa reference the constant is 2.6516458 × 10²⁵ m⁻³ instead; both are exact, since both molar volumes are.