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WALTER A. WOLF Eisenhower College Seneca Falls. New York 13148
Determination of Eutectic Composition: An Empirical Method
+
K. Karunakaran
V.O. Chidambaram College Tuticorin-628003, India Normallv. for a binarv svstem A-B formine a simnle eutedic mixture, t~~eutectict&perature(T,) andlompohtion (X,) can be eranhicallv determined from the measurement of the freezing f i r about 15 to 20 different known compositions of the svstem. A sim~le.less exnensive and ranid . nro. cedure involving only few experimental measurements has been develoued ior the emdrical determination of the cuwctic composition. As T, can he determined from the temperature arrest in the cwling curve of any single unknown composition of the system, knowledge of the freezing point (T.) and the mold freezing point depression constant &(a)) of component A, allows calculation of the mole fraction of A in the eutectic mixture Log X d n ) = -(T.2M./2303K,(o)) [ ( l / T , )- ( l / T . ) ]
(1)
where Ma is the molecular weight of A. From a similar expression for component B, X,(b) may he calculated. The empirical eutectic composition, X., in terms of the mole fraction of A, is then obtained from the mean. X , = [X&)
+ ( 1 - X.(b))1/2
(2)
For a number of simple eutectic systems involving mostly the organic compounds like naphthalene, biphenyl, p-dichlorobenzene etc., the values of X, thus obtained are found to be within f0.02 of the literature values. A more detailed description of the method and a table of results may be obtained upon request.
Chemical Equilibrium: Analogles that Separate the Mathematics from the Chemistry Ronald A. DeLorenzo Middle Georgia College Cochran, 31014 For many freshmen chemistry students the most difficult problems are those associated with chemical equilibrium. Several of the mathematical concepts encountered in chemical equilibrium are relatively simple when isolated from the chemistry. Students become confused when these mathematical concepts are presented as an integral part of the chemical problem because it is the chemistry itself with which they are uneasy. The analogies discussed in this paper accomplish two goals. First, they convince the student that the mathematics is not difficult, and, second, they give further insight into the chemical nature of the equilibrium prohlems. Consider the problem of finding the percent dissociation of a 0.1 N HA solution whose pH = 4, and contrast this with the problem of finding the percent of marriages that have 678 1 Journal of Chemical Education
broken up knowing that there were initially ten couples and that four divorced men were later found. The second of these two problems, along with the "equilibrium" MW = M W, should be discussed initially. Students realize there is little difference between the breaking up of couple MW and the dissociation of acid HA. The mathematics of both ~rohlems is identical. Similar approaches may be taken with problems involving the determination of eauilihrium constants, common ion effects, buffers, and oiher equilibrium calculations. Presented at the 1976 American Chemical Society Southeast Regional Meeting, Gatlinburg, Tennessee.
Atomic Wave Functions Noel Fitzpatrick Owen Wilson Uniuersity College Belfield, Dublin 4, Ireland Insight into the bonding in molecules can be obtained by considering the behavior of atomic one-electron functions. Many facets of elementary chemistry are explained in terms of the shape of orbitals, yet accurate plots are not easily available. of wave functions drawn usina a A booklet giving.. maphs .. . Calcomp plo
