Molar refractivity
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Molar refractivity is a measure of the volume occupied by an atom or group and is dependent on the temperature, the index of refraction, and the pressure.
One form of the Lorentz-Lorenz formula (also known as the Clausius-Mossotti equation) gives the molar refractivity of a dilute gas as <math>
A = \frac{R T (n^2 - 1)}{3p}
<math>
where <math>R<math> is the universal gas constant, <math>T<math> is the temperature, <math>n<math> is the index of refraction, and <math>p<math> is the pressure. In SI units, <math>R<math> has units of J mol-1 K-1, <math>T<math> has units K, <math>n<math> has no units, and <math>p<math> has units of Pa, so the units of <math>A<math> are m3 mol-1. Therefore, the molar refractivity is the volume of the substance (in cubic meters) taken up by each mole of that substance.