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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.

Additional recommended knowledge

Recognize and detect the effects of electrostatic charges on your balance

Essential Laboratory Skills Guide

Daily Visual Balance Check

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.