LETTERS dar Weights-Absc
dative?
To the Editor: I enjoyed reading the letter from Thomas B. Coolidge which appeared in the March issue of the JOURNAL OF CHEMICAL EDUCATION, but feel that additional comment is necessary. I agree with Professor Coolidge that clearly defined terms consisting of properly assigned numerical values and precise dimensions are of great importance in the study of chemistry or of any other science. However, I do not agree that his definition of molecular weight is the only definition or necessarily the best. A good definition should be as fundamental as possible, and should not rely on any particular 'system of physical dimensions. To my mind there are two meanings to the term "molecular weight," and we should differentiate them as follows: 1. Absolute molecular weight or the mass of an individual molecule. 2. Relative molecular weight or a relationship hetween masses of equal numbers of molecules. I would like to state my concept of relative molecular weight by the following definition: If n M , is the mass of n molecules of a substance A, and nMo, is the mass of n molecules of diatomic oxygen, then the relative molecular weight of A, rMA,is given by
is a relative number which furnishes a correspondence between masses of equal numbers of molecules. A convenient unit of mass used in chemical calculations is the mass mol or, as it is generally called, the weight mol or weight molecular weight. If U, is the unit of mass employed, the mass mol of a substance A, MMA, would be defined by MMA 5 U,rMa
Dimensions [Mass]
It has been concluded from experimental evidence that for any choice of a mass unit, such as gram, pound, ton, etc., the mass mol consists of a definite (not arbitrary) number of molecules. This number, which I shall designate by N U m , furnishes us with a connecting link between relative molecular weight and absolute molecular weight. If aM* is the absolute molecular weight, then MM* = D MA = Nu,
Nu,
Dimensions [ M w ]
The definitions given above certainly work, and I feel that they are clear and precise. WESLEYE. BRITTW U N I V E R ~ ~ TOP Y COLORADO
BOULDER. COLORADO
Adventures of Kekulb-Brooklyn
Style
To the Editor: In my class in organic chemistry there is a boy with where 32 is an arbitrary but convenient dimensionless a very fine sense of humor, and he is not going to let the austere subject of organic get him down. unit. On a term test the other day, the class was asked to From the above definition, it is apparent that relative molecular weight is neither a mass nor a weight, but discuss "The Benzene Problem." This student's rMA 3 32nMa/nMq
[Dimensionless]
