Here's why:

* Refractive index (n), which is a property of a material, is the ratio of the speed of light in a vacuum to the speed of light in that material. It's a dimensionless quantity as it's a ratio of two speeds.

* Molecular refractive index (MR) is calculated using the molar mass (g/mol), density (g/mL), and refractive index (dimensionless).

* The formula for MR involves a combination of these quantities in a way that cancels out the units, leaving it unitless.

Therefore, although the components used in the calculation of MR have units, the final value of MR itself is unitless.