2-Handouts_HO-17-20 - 17 Molar mass There are different...

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17 Molar mass: There are different ways to report a molar mass like (a) Number average molar mass, (b) mass average molar mass, (c) Viscosity average molar mass, (d) Z- Average molar mass Number Average Molar Mass Colligative properties (like vapor pressure lowering, freezing point depression, osmotic pressure etc) depend on the number of molecules and not on their size. M n = Σ x i M i = (Eq 1) Σ N i M i Σ N i i where, x i = Number fraction = mole fraction = Σ N i N i Mass Average Molar Mass Mass average molar mass depends on size (mass/volume) as well as number of molecules. Properties like diffusion, sedimentation, light scattering depends both on size and mass of the molecules. M w = Σ y i M i = = (Eq 2)) Σ w i M i Σ w i i Σ N i M i 2 Σ N i M i i where, y i = mass or weight fraction = Σ w i w i i w i = N i M i Z-Average Molar Mass The molar mass depends both on size and mass of the molecules. However, the contribution of the mass of the particle are weighted further in this type of molar mass. M z can be determined by ultracentrifugation technique. M z = (Eq 3 Σ N i M i 3 ) Σ N i M i 2 i
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