Refractive Index and Reciprocal of Carbon Number

FIGURE 4.5 Refractive index versus l/(c = z) for homologous series of hydrocarbons. Source J. Smittenberg and D. Mulder, Relations Between Refraction, Density and Structure of Series of Homologous Hydrocarbons. II. Refraction and Density at 20°C of n-Alkyl-cyclopentanes, -cyclohexanes and -benzenes, Recueil des travaux chimiques, vol. 67, pp. 826-838 (1948). 

Very similar straight lines and intercept values are also obtained if the data are plotted versus 1/M (where M is the molecular weight). From the equations above we get 

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