Level: introductory
Reference: Michael Faraday, "Experimental Researches in Electricity. Seventh Series," Philosophical Transactions of the Royal Society 124, 77-122 (1834).
Notes: Michael Faraday (1791-1867) was one of the greatest experimental scientists of the 19th century. He is best known for contributions made to the study of electricity in both its physical and chemical aspects (electromagnetic induction, for instance, and laws of electrolysis). He also carried out important experiments involving the rotation of plane-polarized light in magnetic fields and the liquefaction of chlorine. He was well respected as both a researcher and a lecturer at the Royal Institution of Great Britain.
In the paper on which these exercises are based, Faraday introduced the notion of electrochemical equivalence. He demonstrated the notion's utility in determining "equivalent weights" with the data used in the exercises. In the same paper, Faraday introduced the terms electrode, anode, cathode, and electrolysis. Exercise 1 involves calculations similar but not identical to those Faraday carried out. For one thing, the mole had not been defined in Faraday's time. Also, the "equivalent weight" for water was 9, not 18, so Faraday arrived at a different value for the equivalent weight of tin. Exercise 2 uses information unknown to Faraday to resolve one of the difficulties in the determination of atomic and molecular weights in the 19th century. A variety of experimental methods could generate mass ratios of substances (such as 3.2 to 0.49742 in this problem); what was not known was whether both of those masses referred to the same number of molecules. We see in exercise 2 that the mass of water and that of tin refer to the same number of molecules and atoms respectively (but to twice that number of electrons).
In exercise 1b, I did not follow standard rules for significant figures. Those rules were not uniformly followed in the early 19th century, and it is clear to me from other numbers reported in the paper that Faraday did not follow them. I mention this because it is quite possible that the 3.2 grains of tin reported was actually known to more than two significant figures. So I have no compuction reporting the molar mass of tin to three figures.
Solutions: To download solutions, go to:
http://web.lemoyne.edu/giunta/classicalcs/faraday.doc
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