Molar refractivity, A , is a measure of the total polarizability of a mole of a substance and is dependent on the temperature, the index of refraction, and the pressure.
The molar refractivity is defined as
A = 4 π 3 N A α , where N A ≈ 6.022 × 10 23 is the Avogadro constant and α is the mean polarizability of a molecule.
Substituting the molar refractivity into the Lorentz-Lorenz formula gives
A = R T p n 2 − 1 n 2 + 2 For a gas, n 2 ≈ 1 , so the molar refractivity can be approximated by
A = R T p n 2 − 1 3 . In SI units, R has units of J mol−1 K−1, T has units K, n has no units, and p has units of Pa, so the units of A are m3 mol−1.
In terms of density, ρ molecular weight, M it can be shown that:
A = M ρ n 2 − 1 n 2 + 2 ≈ M ρ n 2 − 1 3 . 