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 (5.12):