the case of step-growth polymerization, as per the Carothers' equation. Mass average molar mass The mass average molar mass (often loosely termed weight average molar mass) is another way of describing the molar mass of a polymer. Some properties are dependent on molecular size, so a larger molecule will have a larger contribution than a smaller molecule. The mass average molar mass is calculated by where is the number of molecules of molecular mass . The mass average molecular mass can be determined by static light scattering, small angle neutron scattering, X-ray scattering, and sedimentation velocity. The ratio of the mass average to the number average is called the dispersity or the polydispersity index. The mass-average molecular mass, , is also related to the fractional monomer conversion, , in step-growth polymerization (for the simplest case of linear polymers formed from two monomers in equimolar quantities) as per Carothers' equation: where is the molecular mass of the repeating unit. Z-average molar mass The z-average molar mass is the third moment or third power average molar mass, which is calculated by The z-average molar mass can be determined with ultracentrifugation. The melt elasticity of a polymer is dependent on . See also Distribution function Flory–Schulz distribution Schulz-Zimm distribution Mass distribution Sedimentation References Polymer chemistry