W. H. Slabaugh Oregon State University Corvallis 97331
Avogadro's Number by Four Methods
I n the spring term of our honors general chemistry, students are given the choice of designing and performing a few experiments in depth from a list of some twenty suggestions. The students, after a term of conventional qualitative analysis and a term of quantitative analysis (using the Waser manual), are usually up to the point where they can push ahead on their own in designing experiments to answer the questions they themselves raise. Experiments are essentially limited to the equipment available in the freshman laboratory, and frequently, significant results are achieved. This past spring was no exception. Two students, Greg Flanders and Jay Robertson, chose to compare four methods of finding Avogadro's number. The Perrin method,' the spreading of a monomolecular layer2, the electrolysis of water, and electroplating were the four bases on which Avogadro's number was determined. I n the students' reports on each of these experiments, an average of eight references to the literature were cited, and each report was about 10 typed pages in length. This represents the level of thoroughness with which the work was performed. The first two methods have been amply described in THIS JOURNAL. However, a brief description of the electrochemical methods may be of interest. I n the electroplating method, four electrolysis cells were operated in series and triplicate runs were made. Solutions of approximately 1 M Cu(II), Zn(II), Ag(I), and Pb(I1) served as the electrolytes along with electrodes of the corresponding metals. No acid was added to the electrolytes as has been s ~ g g e s t e d . ~ A current of 50 ma a t 9.4 v was passed through the series of cells for 30 min. The electrodes were weighed before and after the electrolysis, and these wcight changes along with the electrical current data gave the information from which values of Avogadro's number were calculated. An average value of 5.960 X loZawith a mcan deviation of 0.030 X loz3was obtained for the Cu and Ag electrodes. Pb's deviation in precision was about 0.1 X loza,and the Zn electrode was quite unsatisfactory. I n one of the runs, Cu/Cu(II) cells were placed a t the two ends of the series. Identical results proved that the position in the series is unimportant. I n the electrolysis of water, a conventional Hoffman
40 / Journal of Chemkul Educafion
apparatus was used.4 Results of four rnns gave a value of 5.63 X loz310.03 for Avogadro's number. A greatly simplified electrolysis device was explored by another group of students who became interested in the experiment. A wide-mouth bottle fitted with a 3-hole rubber stopper accommodated two platinum electrodes and a vent that was essentially a water vapor trap containing anhydrous CaC12. An electrolyte of 1 M Na&O* was placed in the bottle, and an electrical current was passed through the cell for a measured time. The loss of weight by the bottle and its contents represents the amount of evolved H, and Oz which escapes through the vent. Results of this procedure gave values of Avogadro's number that were about 2% high, and the students hypothesized that the inefficient water vapor trap was the prime cause of the error. The values for Avogadro's number reported by the students are:
~lectro&atingof Cu and Ag
5.96 X 10%'
I n their summary, the students expressed a high level of enthusiasm in finding an answer to their question of how accurately Avogadro's number can be found with relatively simple equipment. They also enumerated the variety of concepts and new techniques encountered. Perrin's method introduced them to colloidal dispersions, Brownian movement, gas laws applied to settling in dispersions, and particle size determination. The monomolecular layer method involves spreading and other surface phenomena, and solution concentrations. Electrolysis and electroplating pointed up the unit charge, vapor pressure, density, electrical circuitry, atomic weights, and several more. To the instiuctor, it was refreshing to follow the students during the experiments when they became aware that Avogadro's number is a fundamental quantity and not merely a few digits to be memorized. a
SLABAUGH, W. H., J. CHEM.EDUC.,42,471 (1965). KING,CARROLL L., AND NEILSEN,E. K., J. CHEM.EDUC.,
35, 198 (1958). a SULCOSKI, J. W., AND RABH,F. J., "Experiments in Physics," Bwgess Publishing Co., Minneapolis, 1966, p. 113. 'JOHNSTON, J. E., J. CHEM.EDUC.,16, 334 (1939).
