Polymer samples tend to be mixtures of molecules of different mass (i.e. they are
polydisperse). The average quantity that represents the molar mass depends on how the average
is calculated. The quantity calculated with the Mark-Houwink equation is the viscosity-average
molar mass, Mv. Another measure is the number-average molar mass, Mn. The relationship
of these two depends on the distribution of molecules of different masses in a sample. For a log
normal distribution one has the relation in equation