letters Vacuum Glasswere Safefy
To the Editor: I would like to make some brief comments about thk article which appeared on page 790 of the November, 1969 issue of THIS JOURNAL entitled, "A Vacuum Sublimation Apparatus for the Undergraduate Organic Laboratory." Although this article describes a useful and novel addition to the undergraduate laboratory, I feel i t represents a real safety hazard. In the same on page A818 under safety, your issue of the JOURNAL editor states that one should "apply vacuum only to glassware made for such services." The article by Dr. Ilrause, however, suggests that the undergraduates work with beakers and applies a vacuum to them. This represents several hazards
as described in t,he article bas never suffered implosion even when heated to 150°C at lo-%mm. A free flame has never been used as the source of heat. The apparatus assembled from a 300-ml beaker has imploded in use by a few of our students; but, with the beaker wrapped in tape, i t was in each case the bottom which imploded to produce a hole the size of a quarter with no other damage being done. I n summary, if normal safety precautions such as the following are observed, we believe there is no extraordinary danger to the student using this apparatus.
4. Avoid mechanical shock to glassware under vacuum
1. Beakem are flabbottomed and have more strain and less safety than round-bottom vessels under vacuum. 2. Beakers (especially if used) may have star cracks or hair m r k s which eould easilv fail and cause an imdasion. vacuum.
Finally, I would like to make one comment about a misconception that exists in the mind's of many chemists (the Author of this article is certainly not one of these) regarding so-called "moderate" and "high vacuum!' Many chemists will use questionable vacuumware with an aspirator at 18 mm that they would not dream of using at 10-J mm. It must be stressed that the danger from implosion is about the same a t both pressures since the absolute pressure differential on the wall of the vessel is only slightly different under these two conditions.
To the Editor: Professor Harry Rubinstein of Lowell Technological Institute has questioned the safety of the apparatus described by us, [46, 790 (1969)l. In writing the article no attempt was made to stress t,he observation of the common safety rules everyone should follow when working with any glassware under vacuum. For example, each of the beakers used was wrapped with tape to prevent flying glassware in the event of implosion, and each apparatus was tested a t mm pressure behind a safety shield before use by any of our students. Finally, the students were cautioned about the dangers of vacuum opcmtions as part of a general lecture we give on safety in the laboratory. Following t,he above mentioned precautions, in use by us, the apparatus assembled from a 100-ml beaker
Special Tmining for Indian Students
To the Editor: I am sure that the article by Vold and Mukerjee that appeared in the December issue of THIS JOURNAL will prove of very great help to many chemistry departments that are considering applications from Indian students for admission to graduate schools in the U S . Their detailed analysis of the Indian educational system, the summary of performance by Indian students a t U.S.C., and the guidance from the performance on the advanced chemistry section of the Graduate Record Examination seem particularly valuable to me. There are a few comments that I would like to add that may make the transition from one educational system to another a hit easier for the student. At the present time the typical American undergraduate has much greater opportunity to use sophisticated equip ment in his laboratory program than does his Indian counterpart. I n addition he often has had more experience in building and assembling apparatus. It would be a great service to Indian students if this difference in training were recognized, and special orientation programs might be instituted during the first year of graduate training for these students. Not only might it make the challenge of graduate work less formidable, but I know it would be of immense value to Indian students when they return home.
Volume
47,Number 4, April 1970 / 323
The Mole Again!
To the Editor: The "Chemical Query" in Tms JOURNAL [45, 718 (1968) I about the question whether the mole is a number or a weight shows that the mole concept is still a problem when adhering to strict formalism and yet not confusing beginning students. The answer given that neither alternative is appropriate is quite right: the official definiti~n',~ clearly says that the mole is an amount of substance. Most difficulties arise indeed from the fact that this amount of substance is not characterized by its weight, nor by its mass, but only by jts number of particles (elementary units). Therefore, if somebody wants to provide himself with a mole of a certain substance, he fundamentally has to start counting particles or units from a stock of that substance until be has obtained a number of them equal to Avogadro's number (NA). Surely, counting of particles or units is not usual in chemistry; as a rule the chemist wants to get a certain amount of a substance by weighing. This may give some validity to the idea that the mole concept could unify a collection of earlier defined terms: gram-atom, gram-molecule, or gram-formula. These are also amounts of substance, but now indeed characterized by means of their mass: A or M gram for substances with atomic mass A or molecular (formula) mass M, respectively. As the atomic mass of 12Cis defined to be exactly 12, a gram-atom of lZCis exactly 12 g and, therefore, the same amount of substance as a mole of lZC. A gramatom or gram-molecule (gram-formula) of every other substance is defined as the amount of that substance having a mass of A or M grams. To provide himself with a gram-atom or a gram-molecule (gram-formula) a chemist bas to weigh (the usual way of determining mass in chemistry) until he has A or M grams. The "Chemical Queries" answer, saying that it should be correct to define the mole only to students in terms
' Document UIP 11 (SUN 65-3) "Symbols, Units and Nomenclature in Physics," published by IUPAP 1965, section 9.1.7: "The mole is defined as the amount of substance of a system, which contains the same number of molecules (or ions, or atoms, or electrons, as the case may be) as there are atoms in exactly 12 gramme of the pure carbon nuclide "C." a Professor Dr. F. Jellinek of this University informed me that this definition is about to be somewhat modified according to the finally revised (1969) tentative version of Document STU/C manual of Symbols and Terminology for Physicochemical Quantities and Units" from IUPAC, section 3.6: "The mole is the amount of substance of a system, which contains as many elementary units as there are carbon atoms in 0.012 kilogramme of carbon-12. The elementary unit must be specified and may be an atom, a molecule, an ion, an electron, etc., or a specified group of such entities." This modification will solve most problems connected with using the mole concept as shown at the end of this letter, but not those encountered when presenting it. a Strictly, the IUPAP definition of the mole is inappropriate here because a gram-formula of NaCl contains NA ions NaC and N A ions C1-, thus 2N.4 particles! This incorrectness as well as the chlorine ambiguity will disappear by using the IUPAC modified definition. ' I t does not seem advisable to write "a mole of carbon dioxide molecules" as suggested in the "Chemical Queries" answer; this phase transforms a mole into a number.
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Journal of Chemical Education
of mass because it is usually specified in this way, is typical of the very source of trouble: Both moles on the one side and gram-atoms, gram-molecules, or gram-formulas on the other side have definitions of equal formalism but with differentstarting-points. We neglect that difference when we simply declare them to be identical. This is only evident of I2C; in presenting that identity for all other substances we first have to show its validity! This can be done very simply when we start with pointing out that Avogadro's number is the ratio b e tween two mass units: 1 g (I/,, of the mass of 12 g 12C) and 1 amu ('/I? of the mass of 1 atom lzC.) (We even can take this as a handy definition of that number). Then we only have to multiply by A or M Ig=N~Xlamu
$ } x I ~ = N ~AMxamu amu { gram-atom maas of 1 {gram-molecde =:N* X mass of 1
t,"gCde
~ ~ ~ ~ , " , " =i &mass , eof E,"z&,es mass of 1 {gram-at0m gram-molecule = mass of 1mole mass of 1
NA
After having learned to appreciate this the students may be able to choose from the words mole, gram-atom, gram-molecule or gram-formula that one which is most appropriate in a given situation, e.g. When reacting with sodium, 1 gram-molecule of chlorine takes up B moles of electrons to form S moles/gmm-formulas of sodium chloride.
In the case of chlorine the word mole is ambiguous (atoms or molecules must be specified). In the case of sodium chloride either expression can be used.a In the case of electrons the word mole can and should be used though it cannot strictly be considered as an amount of a real substanoe! The official definition of the mole, however, allows the word mole in these and similar cases, where the mole concept is used as a number without being it. Apparently, the convenience for those who want to use the concept as a quantity bas been considered as a matter of more importance than strict formalism. The IUPAC modified definition of the mole2seems to solve most present difficulties connected with using the word mole, provided we train our students (and ourselves!) always to specify the elementary units When reacting with sodium 1 mole of Cl3 takes up 2 moles of e-to form 2 moles of NaCl.
So the students can be taught to use only the word mole, but they should be aware that as soon as they think to cha~acterizea mole of COZ' as 44 g of that substance, they are in fact characterizing a gram-molecule of CO?. Therefore, when presenting the mole concept, the general identity of its mass with that of a gramatom, gram-molecule or gram-formula should be emphasized.
