I have a PVA bottle, which says that the molecular weight (Mw) of PVA is between 85000 - 124000.

I know that this means that there are repeated chains of the PVA monomer, but what are the units to the molecular weight (of PVA). Is it $\frac{\mathrm{g}}{\mathrm{mol}}$? I am having trouble grasping this concept. Or does 85000 - 124000 mean the number of monomers in the chain of this PVA?


1 Answer 1


First of all $M_w$ is not the number of monomers in a chain of PVA. The number of monomers in a chain is called the degree of polymerization. $\overline {M_w}$ is the weight average molecular weight. It is a way of determining the molecular mass of a polymer. As polymer molecules have different chain lengths, the average molecular mass will depend on the method of averaging. For example, we can use the number average molecular weight which is based on different type of averaging.

As for the unit, it's as you mentioned: g/mol. In biochemistry, they use the dalton (Da) as a mass unit for biopolymers (1 $N_a.$Da= g/mol, where $N_a.$ is the Avogadro number). So, when we say a protein has a molecular weight of 64000 g·mol$^{−1}$, it has also a mass of 64 kDa.

P.S. To be rigorous, we have to say ** molecular mass** instead of ** molecular weight**. But this was an old way of naming things.

  • $\begingroup$ So is the molecular weight in case of PVA between 85000 - 124000 g/mol? $\endgroup$ Jul 27, 2015 at 18:38
  • $\begingroup$ @chemistry.stackexchange.com/users/17750/user5139637 Yes, it is. $\endgroup$ Jul 27, 2015 at 20:36
  • $\begingroup$ $1\ \mathrm{Da} = 1\ \mathrm{u} = 1.660\,538\,921(73) \times 10^{-27}\ \mathrm{kg}$ $\endgroup$
    – user7951
    Aug 21, 2015 at 18:33
  • $\begingroup$ @Loong you are right. But when we say a protein with a molecular weight of 64000 g·mol−1, it has also a mass of 64 kDa. $\endgroup$ Aug 22, 2015 at 3:52

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