Problems Atomic weights

Exactly the same procedure can be followed to define the free energy of a dislocation core. It should be surrounded by a box, the terminating planes of which can be dealt with exactly as above. Special attention has to be given to the atoms at the comers of the box, but this presents no particular problems their weights are simply a oroduct of the weights generated by the planar terminations which they share. 

One of the problems that further compounded these difficulties was the fact that many of the atomic weights with which Mendeleev was operating—that is, aside... 

The problem with triads, as well as the other important numerical hypothesis due to Prout, is easy to discern in retrospect. It is simply that atomic weight, which both concepts draw upon, is not the most fundamental quantity that can be used to systematize the elements. The atomic weight of any element depends on the particular geological origin of the sample examined. In addition, the atomic weight of any particular element is an average of several isotopes of the particular element. 

The designers of the lecture room were, of course, proved correct. Only a few years later a systematic order was, indeed, recognized. An extraordinary double discovery was made in 1869. The German chemist Julius Lothar Meyer (1830-1895) noticed a remarkable periodicity during his rigorous scientific analysis of the atomic weights and volumes. He remained content with only a mild curiosity in this realization, as his interests lay primarily in physicochemical problems. He was objective and driven only by facts he was wary of hypotheses... 

Ans. There are 6.02 X 10 i atoms in 1.00 mol Na (Avogadro s number). There is 23.0 g of Na in LOO mol Na (equal to the atomic weight in grams). This problem requires use of two of the most important conversion factors involving moles. Note which one is used with masses and which one is used with numbers of atoms (or molecules of formula units). With numbers of atoms, molecules, or formula units, use Avogadro s number with mass or weight use the formula weight. 

Ans. The atomic weight. We are solving for a formula, which is a ratio of atoms. This type of problem has nothing to do with oxygen gas, 02. 

To get the final answer, we must divide the energy released by the moles of glucose. We find the moles of glucose from the mass given in the problem and the molar mass determined from the atomic weights of the elements. 

Theodore William Richards and the Atomic Weight Problem. Science 164, 647-51 (1969). 

Problem 7 Listed in the following chart are the atomic weights (measured in atomic mass units) for natural silver and its two isotopes. Use this data to calculate the percentage of silver-109 in the natural mixture. 

Historians have been much exercised to explain this half-century delay in the recognition of the application of Avogadro s principle for the solution of the atomic weight problem. It is not a part of this story to attempt a deeper explanation here, but it is appropriate to ask in a general way if anyone could have done earlier what Cannizzaro did about i860. For our purposes it is sufficient to offer two very broad general circumstances within which the many detailed factors may be subsumed. 

Philosophers of antiquity had offered composition and structure as alternative explanations for chemical behavior. These were thought to be exclusive views, but they became one when joined by Daltons inspired conception of a quantized atomic weight. By permanently solving the problem of composition, Daltons theory also made possible the open-ended exploitation of molecular structure, the central theme of organic chemistry. 

The results of analysis usually are given in percentages of the constituents, not in terms of the amounts actually weighed (as in the last problem). This permits comparison of results from different experiments. You recall that "percent" means "per 100." We can say, then, that if we have 100 g of the compound, 90.6 g is Pb and 9.4g is O. We can also say that if we have 100 d of compound, 90.6 d is Pb and 9.4 d is O. Using Dalton atomic weights we can easily find the number of atoms in 100 d of compound, as follows. 

Notf Problems concerning the determination of appioximate atomic weights by the rule of Dulong and Petit may be found in Chapter 14... 

We will finesse here the long and important struggle of getting a truly self-consistent table of atomic weights. If you are interested in the complex history of this problem and the clear solution to it proposed by S. Cannizzaro in 1860, there are many accounts available in books on the history of chemistry. One example is J. R. Partington, A History of Chemistry, Vol. IV, Macmillan, London, 1964. Relative atomic weights now are based on 12C =12 (exactly). 

Named after its discoverer, the same Cannizzaro who, in 1860, made an enormous contribution to the problem of obtaining self-consistent atomic weights (Section 1-1). 

PROBLEMS In each of the compounds below, find the stereocenter, and label the four groups from I to 4 using the system of priorities based on atomic weight. 

The ultimate link to the SI system is currently possible in some cases only by using pure substances (e.g. elements) and their corresponding atomic weights however, a more straightforward way is to use direct methods like coulometry, which eliminate the problems associated with the dissolution step. Their use can fulfil both the role of a standard (they incorporate the unit mole into the measurements) and the role of measurement capability. 

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