Molecular Mass


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Molecular Mass

 

(or molecular weight), the value of the mass of a molecule, expressed in atomic mass units. In practice the molecular mass is equal to the sum of the masses of all the atoms comprising the molecule; multiplication of the molecular mass by the accepted value of the atomic mass unit, (1.66043 ± 0.00031) X 10−24g, gives the mass of the molecule in grams.

The concept of molecular mass firmly entered science after the differences between the atom and the molecule were clearly formulated as a result of the work of S. Cannizzaro, who developed the ideas of A. Avogadro. The discovery by F. Soddy of the phenomenon of isotopy and the elaboration by F. Aston of the mass-spectrometry method nf mass determination contributed to a more precise definition of the concept of molecular mass.

The concept of molecular mass is closely related to the definition of the molecule, but it is applicable not only to substances whose molecules exist separately (gases, vapors, certain liquids and solutions, and molecular crystals) but also to other cases (such as ionic crystals).

The average mass of molecules of a given substance, taking into consideration the percentage of isotopes of all elements comprising it, is often taken as the molecular mass. It is some-times defined not for an individual substance but rather for a mixture of various substances of known composition. Thus, the “effective” molecular mass of air may be considered to be 29.

Molecular mass is one of the most important constants characterizing individual substances. The molecular masses of different substances differ sharply. For example, the molecular masses of hydrogen, carbon dioxide, sucrose, and the hormone insulin are 2.016, 44.01, 342.296, and about 6,000, respectively. The molecular masses of some biopolymers (proteins and nucleic acids) reach many millions or even several billions. The values of molecular mass are widely used in various calculations in chemistry, physics, and engineering. Knowledge of the molecular mass automatically gives the value of the gram-molecule (mole) and makes possible calculation of the density of a gas (or vapor) and the molar concentration (molarity) of a substance in solution and the determination of the true formula of a com-pound from data on its composition.

The experimental methods of determining molecular mass have been developed mainly for gases (vapors) and solutions. Avogadro’s law underlies the definition of the molecular mass of gases or vapors. It is known that the volume of 1 mole of gas (vapor) is about 22.4 liters under normal conditions (0°C and 1 atmosphere). Therefore, by determining the density of the gas or vapor it is possible to find the number of moles and consequently to determine the molecular mass. In the case of solutions, cryoscopic and ebulliometric methods are most often used to deter-mine molecular mass. Experimental methods provide data on the average value of the molecular mass of a substance. The molecular mass of individual molecules can be estimated by means of mass spectrometry.

Molecular mass is an important characteristic of macromolecular compounds (polymers) and defines their physical and technological properties. The macromolecules of polymers are formed by repetition of comparatively simple units (groups of atoms); the number of monomeric units making up various molecules of a given polymeric substance is different, and consequently the molecular mass of the macromolecules of such polymers also differs. Therefore, in characterizing polymers one normally speaks of the average value of molecular mass; this quantity gives an idea of the average number of units in the molecules of the polymer (the degree of polymerization).

The function of molecular mass distribution gives a complete description of the dimensions of a polymeric molecule; this function makes it possible to determine the fraction of molecules (of a certain size) of a given polymeric substance whose molecular masses lie in a given range—from M to( M + ΔM).

In practice the average molecular mass of a polymer is usually determined by investigating its solution by some method. The properties of solutions may depend on the number of molecules present in the solution (it should be noted that molecules that differ in mass behave entirely identically) and on the mass (weight) concentration of the solution (in this case, one large molecule produces the same recorded effect as several small molecules). If the polymer consists of nonidentical molecules, then the average values of molecular mass as measured by different methods will differ. For example, a decrease in the freezing point or an increase in the boiling point of a dilute solution depends only on the number of molecules present and not on their dimensions. Therefore, cryoscopic and ebulliometric methods make it possible to find the average molecular mass of a polymer (the “simple” average). The intensity of the light scattered by a solution of a polymer depends on the mass of the substance present in the solution, and not on the number of molecules. Therefore, a method based on measurement of the intensity of scattered light is used to determine the value of the molecular mass of a polymer, averaged with respect to mass. Other methods (such as sedimentation equilibrium and the viscometric method) make it possible to find other average values for the molecular mass of polymers. By comparing the average values of molecular mass determined by different methods, it is possible to draw a conclusion about the molecular mass distribution. In the simplest case, when the average molecular mass of the polymer coincides with the value of the mass-averaged molecular mass, one may conclude that the polymer consists of identical molecules (that is, it is monodisperse).

REFERENCES

Nekrasov, B. V. Osnovy obshchei khimii, vol. 1. Moscow, 1973.
Guggenheim, E. A., and J. Prue. Fiziko-khimicheskie raschety. Moscow, 1958. (Translated from English.)
Houben-Weyl. Melody organicheskoi khimii, vol. 2. Moscow, 1967.

S. S. BERDONOSOV

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