Correspondence "I
A QUESTION OF PROCEDURE Under this title, Mr. Charles H. Stone,' a clever teacher of chemistry in the English High School of Boston, recommends that the time-honored procedure for computing the smallest possible molecular weight of a compound from its percentage composition be changed. Instead of first finding out how many gram atoms of an element are present in one hundred grams of the original substance and in that way finding the ratio existing between the atoms in a given weight of substance, Mr. Stone first finds the molecular weight on the basis that one atom of some element, chosen a t random, is present in the molecule. Thus if a sub23 X 100 = 53 stance contains 43.4% of sodium, the molecular weight is 43.4 if only one atom of sodium is present in the molecule. According to the conventional method the first step in the reasoning is to say that since 43.4 grams of sodium are present in 100 grams of substance there must 43.4 be - = 1.89 gram atoms of sodium in 100 grams of material. 2.1 -.
Mr. Stone comes in contact with boys who are not as a rule going to study chemistry after he is through with them and it is quite natural with such pupils to attempt to draw analogies between chemical problems and interest problems but there seems to be little choice between the two methods of reasoning and the inference which one draws on reading Mr. Stone's paper, that his method is a more logical one, does not seem to correspond to the facts. It starts with an assumption that is not true in many cases whereas the older method starts with an assumption that is always true.
FURTHER QUESTION In a recent publication of the JOURNAL OF CHEMICAL EDUCATION appeared an article which advocated the teaching of an ingenious, yet circuitous, method for the calculation of empirical formulas in preference 'THISJOURNAL, 5, 465 (Apr., 1928).
to the present usually accepted method. The objection raised against the present method is that the steps in the solution do not seem to parallel those employed in common problems involving percentage and principle. However, if the teacher advises the beginner to conceive of the composition of the compound in terms of grams per 100 for each element when the composition is given in per cent, as do many authors of elementary texts, the logic of the present method will a t once become evident. Furthermore, no confusion will result from any preconceptions about percentage problems because no percentage is involved. The important reason why all authors of chemistry texts adhere to the older method instead of following such a procedure as suggested by Mr. Stone is that the former goes directly to the solution of the problem, while the latter necessitates the calculation of two intermediate sets of figures, R and C, which represent the composition of the substance just as do the original data. The relative amount of work required by the two methods for the derivation of the empirical formula of a compound from possible experimental data can best be illustrated by the example given below.
Carbon Hydrogen Oxygen
0.3768 g. 0.0319 g. 0.0871 g. by difference
A-PRESENT METHOD
0.37 6' 12 0.0319 -1.008 0.0871 -16
= 0.0313 atomic weights
of carbon in 0.4958 g. of substance
- 0.0306 atomic weights of hydrogen in 0.4958 g. of substance - 0 . W544 atomic weights of oxygen in 0.4958 g of substance
Therefore, formula is C6He0.
O-o X-i 1 - O.1757 or 17.57% oxygen 0.4958 12 - X 100 = 15.79 multiple or submultiple of molecular weight which contains 12 g, ar 76 1 atomic weight of carbon 0.0643 X 15.79 = 1.015 g. of hydrogen in 15.79 g. of substance 0.1757 X 15.79 = 2.774 g. of oxygen in 15.79 g. of substance
2 = 1 atomic weights of carbon in 15.79 g. of substance 12 --
1.015 -= 1.007 atomic weights of hydrogen in 15.79 g. of substance 1.008 2'774 - - 0.173 atomic weights of oxygen in 15.79 g. of substance 16
Therefore, formula is GH60.
"THE REPLY COURTEOUS" My small adventure in chemical arithmetic has evoked an unexpected animation from those in "the seats of the mighty." Their animadversions add vivacity to days which otherwise might become jejune. I am inveigled by the criticism offered hut make my first how to Prof. Hall. When I saw the word "clever" in his criticism I was surprised, hut when I looked up the definition in Webster I was overwhelmed. I am not worthy of such a Niagara of commendatory synonyms. I expect this compliment will cost me a cigar when next I meet the Professor. In justice to my school, I must correct two erroneous assumptions. Prof. Hall assumes that the method I put forward is taught in my classes. My article does not say so and it is not true. It has not been thought good pedagogy to teach a method at variance with that in the text. He assumes that the method was evolved to suit the intelligence of the boys with whom I am "in contact and who, as a rule, will not study chemistry further after I get through with them." Good joke, Professor. No doubt when I get through with them some, a t least, will have been permanently cured of the desire to study chemistry any more.
