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The number average molar mass is a way of determining the molecular mass of a polymer.Polymer molecules, even ones of the same type, come in different sizes (chain lengths, for linear polymers), so the average molecular mass will depend on the method of averaging.
Molecular weight (M.W.) (for molecular compounds) and formula weight (F.W.) (for non-molecular compounds), are older terms for what is now more correctly called the relative molar mass (M r). [8] This is a dimensionless quantity (i.e., a pure number, without units) equal to the molar mass divided by the molar mass constant .
Number of atoms Total molecular mass (Da or u) 12 C: 12.0000: 1: 12.0000 1 H: 1.007825: 4: 4.0313 CH 4: ... The average molecular mass is often used for larger ...
where M w is the mass-average molar mass (or molecular weight) and M n is the number-average molar mass (or molecular weight). Pure Appl. Chem. , 2009 , 81(2), 351-353
The molecular mass of a polymer differs from typical molecules, in that polymerization reactions produce a distribution of molecular weights and shapes. The distribution of molecular masses can be summarized by the number-average molecular weight, weight-average molecular weight, and polydispersity .
The number-average molecular weight (M n) and weight-average molecular weight (M w) are most commonly reported. [28] [29] The ratio of these two values (M w / M n) is the dispersity (Đ), which is commonly used to express the width of the molecular weight distribution. [30]
Therefore, average values are given for the molecular weight like the number average, the weight average or the viscosity average molar mass. A measure for the width of the molecular weight distribution is the polydispersity index.
In polymer chemistry, vapor phase osmometry (VPO), also known as vapor-pressure osmometry, is an experimental technique for the determination of a polymer's number average molecular weight, M n. It works by taking advantage of the decrease in vapor pressure that occurs when solutes are added to pure solvent.