LETTERS Local Gas-Volume Relationships
Atomic Weight Unit
T o the Editor: I wish to submit a few comments as to the desirability of giving a local meaning to gas-volume relationships and local values to supplement those normally given only for standard conditions. The "standard conditions" that are used in connection with gas volumes are possible of exact experimental attainment in but few laboratories anywhere in the world. An air pressure exactly equal to 760 mm. of mercury is not of common occurrence even a t sea level and a latitude of 53.5"N. (or halfway up the coast of Labrador), though the Weather Bureau records indicate that a year-round average of that pressure could be expected for such a location. Nor is a temperature of 0°C. a usual or a comfortable one in any laboratory. One could argue, therefore, that the standards that have been chosen are essentially impractical for either laboratory or industrial couditious. Though one may doubt the desirability of setting up new standards that would have some measure of practicality, it is withm the range of easy accomplishment for any school or any industrial establishment to translate such a volume as 22.4 liters or a hydrogen density of 0.09 grams to the corresponding volume or density that each would have under the average year-round pressure for that community and for some temperature possible of easy attainment. For most laboratories a temperature close to 20°C. (68°F.) is maintained throughout a major portion of the year; such a temperature would, therefore, be a desirable temperature standard for such laboratories. Under such local conditions as those described the gram molecular volume will be larger than 22.4, often more than ten per cent larger., Out here a t the base of the Rockies the volume a t our average air pressure and for 20°C. is equal to 28.77 liters-& 28 per cent larger than 22.4 liters. For a gas collected over water the volume becomes 29.56 liters--or 32 per cent larger. We consider it desirable for our classes to calculate such values for our conditions. In quantitative work involving the collection of gases over water the student in general chemistry is permitted to use 29.56 liters as the volume of a gram molecular weight of the gas, without correction for such minor variations of pressure and temperature as may exist; the failure to make such corrections introduces an error that is commonly no greater than 0.5 per cent. The saving of laboratory time, ordinarily spent in making the calculations related to gas-volume corrections, is tremendous in the aggregate. The omission of such calculations serves to focus attention upon the weight-volume relationships of the chemical reaction. K. GORDON IRWIN
To the Editor: In my first semester of teaching, I was impelled by student reaction, or lack of it, to use a name for the atomic weight unit. The students themselves suggested and adopted the terms "hep" and "pip" in two classes. One examination question read, "If an oxygen atom weighs 24 "xyzts," what does a carbon atom weigh?" The answer was almost unanimously 18 "xyzts," not 18. Students want the unit! But why use a meaningless word, such as "pel?" I suggest the derived tesm "atwu" (pronounced at'woo), which stands for ATomic Weight Unit and sounds like it, too. JOHN E. HODGE WESTERNUNIVERSITY KANSAS CITY, KANSAS
Sodium Bromide-Hydrogen B r o m i d e w a t e r To the Editor: In the article "The Three Component System Sodium Bromide-Hydrogen Bromidewater," by O'Brien, Kenny, and Fuxa 0,CHEM.EDUC.,17, 576 (1940)), the isothermal solubility diagram for the system is represented by what appears to be a smooth curve. Since a change in solid phase takes place as the concentration of hydrogen bromide changes, a break in the curve must be present. The data plotted are neither sufficiently accurate nor suEdeutly numerous to show this break &early. Perhaps the authors intended to indicate a break, but i t is definitely not clear in the printed diagram. If, as the paper suggests, a study of this system is used as a laboratory experiment, the instructor should guard against, possible misconception in the minds of the students that the curve is continuous. WILLIAME. CADBURY, JR.
To the Editor: We regret that the diagram in our paper was not sufficiently clear to indicate a break in the solution curve. Actually, there is an inflection in this curve between hydrogen bromide concentrations of 37.21 per cent and 41.38 per cent (NICOLAEV AND RAVICH,J. Gen. Chem. (U.S.S.R.), 1, 787 (1931)). It will be noted, however, that considerably more data than that suggested in the experiment must be taken by the student if he is to find this inflection for himself. In Reference 1of this paper, the page numbers should read 5734, instead of 533-4. S. JAMES O'BRIEN
