R. J. Kokes, M. K. Dorfman, and T. Mathia
The Johns Hopkins University Baltimore, Maryland
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Experiments for general chemistry I
cloud Chamber, Molecular Film, and Atomic Weight of Silver
The first series of experiments deals with fundamental particles, atomic and molecular structure, and atomic weight. A striking introduction to the atomic theory is provided by a cloud chamber, in which the student actually sees the t,racesof elementary particles. By studying a surface film of stearic acid on water,,he then obtains an estimate of the size of individual atoms and molecules, as well as an estimate of the value of Avogadro's number. Following this, the techniques of quantitative analysis are used to determine accurately the atomic weight of an element. The new freshman laboratory course at Johns Hopkins has been described in THIS JOURNAL,39, 16 (1962). Many of the experiments are innovations in an introductory course. This series of articles describes the experimental procedures in some detail.
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The Cloud Chamber
The diffusion cloud chamber has been a popular lecture tool for demonstrations in nuclear chemistry. A model designed for that purpose has been described by Slabaugh'; however, the less elaborate chamber used a t this university more closely resembles that described by K ~ e h n e r . This ~ chamber, shown in Figures 1 and 2, is easy to constmct, operates continuously, and has never failed to give clearly visible tracks. A Pyrex crystallization dish, 3 in. deep and j 3 / 4 in. diameter, is the chamber. A layer of cotton, soaked with the solvent, is held in place in the bottom of the dish by a liner of asbestos paper. Of the solvents
' SLABAUGH, W. H., J. CHEM.EDUC., 32,269 (1955) 29,511 (1952). 'KUEHNER,A.L., J. CHEM.EDUC.,
employed, methanol was found to give the best results. The dish is inverted on a black cloth whieh covers a square of aluminum foil on top of a flat surface formed with powdered dry ice. Another square of foil is placed on top of the chamber and secured with a beaker half-full of t,epid water. The foils are connected through a plug to the 110-volt de outlet; a resistance is eonneeted in series as a safety device. By applying the field and occasionally reversing its direction, it is possible to sweep t,he chamber free of dust particles and excess ions.
Typical Results
Corrections for Georneb!~ Observed dps (disintegrations per second) for a uranyl nitrate crystal in the cloud chamber Corrected for geometrya Observed dps with Geiger counter Corrected for geometrya
3 dps 18 dps 33 dps 160 dps
Self-absorption Observed dps for a whale crystal of uranium salt using Geiger counter Observed dps for the same crystal, crushed
19 dps 47 dps
Student Absorption Estimation Path length of alpha-particle in air, cm 3 1.37 X 10Wa Path length in uranyl nitrate, crn 1.43 X lo-' Path length in aluminum, em Path lmgth in gold, cm 0.02 X lo-' 0.34 X 10-3 Path length in lead, cm
Literature' 2.73 2.99 X lo-' 1.70 X lo-' 0 . 6 4 X lo-' 1.12 X lo-'
Calmdated Decav Rate Estimated "active mass" of uranyl nitrateb 0.006 grams Estimated dps using the isotopic constitution of uranvl nitrate and the knon-n half-lives of u r a 36 dps nium 234. 235, 238 a Corrections for geometry were made by comparing the reg3on in which emitted particles could be viewed or countcd with the spherical region in which particles were actually emitt,ed. The "active mass" is tho mass of that volume a t the surface from whieh emitted particles are not absorbed by the crystal.
Figure 1.
Diffurion cloud chamber.
Soap Film
Figure 2.
Diagram of the diffusion cloud chamber.
The radioactive sources used in the experiment include nranyl nitrat,e crystals, a piece of thorium metal, and an alpha-ray t i p 3 The student places the source on the black clot,h, whieh serves as a background for viewing thc tracks, inverts the chamber, and applies the field. Aft,er a few minutes the cloud forms and, in the darkencd laboratory, the tracks are clearly visible when the chamber is illuminated by a flashlight. The students not only observe the tracks of ionized particles, hut also st,udy properties of t,he radioactive substances. From the number of tracks seen in a section of the chamber in a given time, multiplied by an appropriate geometrical factor, they estimate the rate of dccay, and compare t.his estimate with the rate observed using a Geiger counter. Self-absorption is demonstrated by counting both whole and crushed crystals of uranyl nitratc. Absorption in materials such as gold and aluminum is studied by placing foils of different thickness over the source until no radiation penetrates the foil. The stopping power of these materials is then compared with the stopping power computed from the path length in air.
When the molecules of an insoluble substance have a functional group with a strong affinityfor water, they cover a clean water surface nith a monomolecular film. I n such a film, the molecules are oriented with the "water-soluble md" in the water, either vertically or a t a steep angle to the surface, and are held together laterally by the intermolecular attraction between the long hydrocarbon chains. Once all the snrface is covered by sueh a film, the addition of another drop of the substance causes an oily lens to appear. The study of sueh films has contributed greatly to our knowledge of molecular structure. An rxcellent historical survey of many of the classical experiments may be found in a book by Adam6 In the exprrim~ntperformed at Johns Hopkins the size of a molecule and Avonadro's nnmber are estimated. A similar experiment in an undergraduate laboratory was described by King and Neilsen,' whose procednre is somewhat different from the following: The experiment is performed with a 14-cm watch glass, whieh is washed, rinsed thoroughly, and filled with distilled water. A micro-pipet is made from a medicine dropper bulb and glass tubing drawn into a capillary such that it delivers 100-150 drops/ema of benzene. After careful calibration this pipet is used to add by drops a dilute (0.05 weight %) solution of stearie acid in benzene to the water in the wat,ch glass. At the start,, after the addition of each drop,
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'RRICHTMEYER, R. K., AND KENNARD, E. H., r'Introdwti~nt0 Modern Physics," 4th ed., McGraw-Hill Book Company, Inc., New York, 1947, p. 560. GEIGER,H., Zeil. fur Physik, 8.45, (1921). 'ADAM, 3. K., "The Physics and Chemistry of Surfaces," 1930. The Clsrendon Press. Oxford. Eneland. " KING, L. C., AND NEILSEN,E. K.,J. CHEM. EDUC.,35, 198 (1958).
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the benzene evaporates and leaves a barely visible, incomplete monolayer of stearic acid on the surface. But when the surface is completely covered, the addition of a single drop in excess forms a small lens or globule on the film. It is thus possible to "titrate" the surface with the lens as the indicator of monolayer formation. The student knows the formula for stearic acid, CH3(CHz)&OOH, the concentration of the benzene solution, and the volume delivered by his dropper. He measures the area of the surface film and is told to assume that the film is one molecule thick with the molecules oriented vertically. From this information and the density of the stearic acid, the thickness of the film is calculated and taken to be the length of the stearic acid molecule. The average length calculated from the data of a high school senior who helped develop the experiment is 24 =t5 A. On the basis of the crude assumption that the carbon chain is linear, the student could calculate the diameter of 3 carbon atom; the data mentioned above yield 1.36 A for this quantity. Finally, the student is given the density of diamond and told to estimate Avogadro's number. The choice of a model is left to the students, but most of them choose to regard the crystal as a stack of small cubes, the carbon atoms. The value calculated for Avogadro's number on this basis is 24 X 102s. A more exact calculation of both the carbon atom diameter and Avogadro's number, using the known structure of diamond, yields 1.67 A for the former and 7.3 X loz1 for the latter quantity. The values calculated for Avogadro's number in this experiment are crude. Although most of the students obtain the right order of magnitude, almost all of them report high results. This reflects the major source of error in the experiment, the difficulty is getting an absolutely clean water surface to start with. Never-
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theless, it is felt that this experiment is valuable, for it gives a reasonable estimate of atomic and molecular dimensions, and a very tangible introduction to the concept of intermolecular forces. The Atomic Weight of Silver
The investigation of molecular structure is completed by a quantitative determination of the atomic weight of silver. The method used is standard in quantitative analysis: precipitation of silver chloride by adding silver nitrate solution to a known quantity of sodium chloride, and weighing the dried precipitate. The only innovation is the use of methanol and suction for drying, which reduces the time needed for the experiment without introducing any inaccuracy. The average result obtained by class for the atomic weight was 112.3, with an error of 2%. Unknowns containing mixed soluble chlorides, and given the formula MC1, are distributed, and the atomic weights of the mythical elements M are determined as above. Most of the students do not do as well on the unknowns but errors exceeding 10% are rare. This experiment is included with the others not only t,o provide an introduction to quantitative techniques but also to emphasize that stoichiometry with all of it,sramifications is merely a macroscopic consequence of the microscopic behavior studied in the preceding experiments. Acknowledgment
Grateful acknowledgment is made of a grant from the National Science Foundation for new teaching aids in chemistry, which made the development of these experiments possible. The authors wish to express their thanks to Mr. Carl Roberts and Mr. William Perkins for help in the development and t,esting of these experiments.
