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R. J. Gillespie
McMaster University Hamilton, Ontario, Canada and C. Solornons
Imperial College London, England
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Mo~o~--ANew Concentration Unit
The concentrations of solutions can be expressed in a variety of different ways. Physical chemists generally use one of the following concentration scales: (1) Molality, m, moles of solute per kilogram of solvent. ( 2 ) Molarity, c, males of salute per liter of solution. (3) Mole fraction, z,moles of solute divided by total number of moles in the system.
We have found in our work on solutions in sulfuric acid' and other non-aqueous solvents that it is often convenient to express concentrations in another way, namely, as moles of solute per kilogram of solution, i.e., in mole kg..,,,,..-1 u n i k 2 This unit has not been widely used and has no special name or symbol. We suggest the symbol w, derived from weight concentration, and the name molon, derived from mole/kg solution. There are several advantages in expressing concentrations on this new scale. Concentrations in molon units are independent of the internal state of the system. Mole fractions and molalities suffer from the disadvantage that they can have more than one value in cases, for example, where the solute reacts with, or is solvated by, the solvent or where the solvent is selfdissociated, Thus one can calculate a stoichiometric molality on the assumption that the solute dissolves unchanged or various "true" molalities depending on the particular assumption made concerning the internal state of the system. I n sulfuric acid many substances behave as bases, e.g., acetone ionizes in the following manner?
And it is possible to express the concentration of the solution either in terms of the stoichiometric molality of the acetone (in effect, one assumes that the acetone dissolves unchanged) or in terms of the "true" molality of the hydrogen sulfate (CHa)%COH.HSO+ Moreover since sulfuric acid is appreciably self-dissociated,' the molality of a solute depends on whether or not the
self-dissociation species are regarded as constituting part of the solvent. Concentrations in molon units are independent of temperature. Volume concentrations such as molarity are independent of the internal state of the system but are temperature dependent and thus have the disadvantage that a given molecular concentration has a different volume concentration a t different temperatures. When solutions are made up by weight their concentrations can only be expressed in molar or other volume units if the densities of the solutions are known. Sometimes, as has been the case for some sulfuric acid solutions, the densities are not known or are not known with very great accuracy. Some workers have attempted to avoid this difficulty caused by the lack of knowledge of the density of a solution by using the density of the pure solvent to calculate an approximate or "pseudo" molarity. As they have often not been careful to distinguish this quantity from the true molarity, confusion has sometimes arisen. This method of expressing concentrations should therefore be avoided. If concentrations are expressed in molon units they are readily converted to molarities if, and when, the densities of the solutions are available, by means of the simple relation: Thus concentrations expressed in molon units have the advantage that they are independent of the internal state of the system and depend only on the weights of the materials used to make up the solution; they are independent of temperature; and they are readily converted to molarities by the above simple relation. GILLESPIE, R. J., AND ROBINSON, E. A,, "The Sulphuric Acid Solvent System," Chap. 9, in "Advances in Inorganic Chemistry and Radiochemistry," Vol. 1, Academic Press, Inc., New York, 1959. GILLESPIE, R. J., Review of Pure and Applied Chemistrg, 9, l(1959). C., J. GILLESPIE,R. J., OUBRIDGE, J. V., AND SOLOMONS, Chem. Soc., 1804 (1957). a GILLESPIE, R. J., AND LEISTEN,J. A., Quart. Rev., 8,40 (1954).
We call readers' attention to an alternate nomenclature, "Molel," proposed for the same unit 36, 616 (1959) by J. M. Thoburn in his article, "Weight Titrations Revived," Tnrs JOURNAL, (December). Readers who have encountered other terms designating the same unit are invited to communicate with the Editor.
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Journal of Chemical Educofion
