Weight and Molar Mass

The term molecular weight is still widely used instead of molar mass , though it can be somewhat misleading. Molecular weight is really a dimensionless quantity. It is the sum of the atomic weights in the formula of the molecule and the atomic weights, in turn, are dimensionless ratios of the masses of the particular atoms to 1/12 of the mass of an atom of the most abundant carbon isotope to which a mass of 12 atomic mass units 

In this book, w e shall retain the term molecular weight because of its widespread use in the polymer literature. However, when using in numerical calculations we shall substitute the molecular weight by its numerically equivalent molar mass (g/mol) to facilitate dimensional balancing. 

---

Calculate the formula weight and molar mass of a compound. 

The term molecular weight was traditionally used instead of molar mass. The terms molecular weight and molar mass mean exactly the same thing. Because the term molar mass more accurately describes the concept, it will be used in this text. 

In many instances, average molar masses and their ratios (i.e. polydisper-sity indices) are insufficient to describe the properties of a polymer and more complete information on the molar mass distribution is required. One way of obtaining this information is to separate (i.e. fractionate) the polymer into a number of fractions each of which has a narrow distribution of molar mass. The weight and molar mass of each polymer fraction are determined and enable the molar mass distribution to be constructed in the form of a histogram. However, such procedures are rarely used nowadays because much more rapid and powerful methods of size-exclusion chromatography (Section 3.17) are available for determining molar mass distributions. Nevertheless, fractionation itself is still practised, often for purposes of purification, and will be considered here in some detail because it introduces the important topic of phase-separation behaviour of polymers. 

Note that when using metric units, atomic weight, molecular weight, and molar mass are expressed in grams per mole (g/mol) and then the mass of the species comes out in grams (g). When looking at the units, it makes sense ... 

---