# Molar refractivity

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 [itex]

 A = \frac{R T (n^2 - 1)}{3p}


[itex]

where [itex]R[itex] is the universal gas constant, [itex]T[itex] is the temperature, [itex]n[itex] is the index of refraction, and [itex]p[itex] is the pressure. In SI units, [itex]R[itex] has units of J mol-1 K-1, [itex]T[itex] has units K, [itex]n[itex] has no units, and [itex]p[itex] has units of Pa, so the units of [itex]A[itex] 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.

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