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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.

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One form of the Lorentz-Lorenz formula (also known as the Clausius-Mossotti equation) gives the molar refractivity of a dilute gas as $A = \frac{R T (n^2 - 1)}{3p}$

where

R

is the universal gas constant

T

is the temperature

n

is the index of refraction

p

is the pressure

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. Therefore, the molar refractivity is the volume of the substance (in cubic meters) taken up by each mole of that substance.