John H. Wolfenden
Dartmouth College Hanover, New Hampshire
Two Student Experiments on Chemical Equilibrium
T h e conventionaI laboratory course in physical chemistry includes and may even consist exclusively of a sequence of carefully documented experiments. Unless the documentation is rather detailed, the experiment is likely to be very timeconsuming or to lead to results regrettably divergent from theoretical expectation. There is little room for Cortez or wild surmise in such a sequence and accordingly the interspersion of an occasional experimental "problem" or "practical examination" can he a welcome challenge to the student. My own practice is to give two or three such examinations in the course of two terms of physical chemistry. Every student works independently during the examination and is expected to present a written report on his work a t the end of three or four hours. A description follows of two of the experimental problems that I have used for such examinations. The first of these, on the ammonia silver complex, was a part of the Oxford University final A.B. examination in 1924; the second, a reconstruction of the classical work of Nernst on the solubility product, is (like the first) included in somewhat different form in "Problems of Practical Physical Chemistry" by the late F. A. Philbrick (J. M. Dent, London, 1931) a useful little book now unfortunately out of print. The ~ s t a b k h m e n tof the Formula of a Complex Ion
The instructions given to each student are as follows: Add to 20 ml of M/100 silver nitrate solution 10, 20, and 30 ml of 2 M ammonia salution. Make up each solution up to 100 ml with distilled water and then find what volume of M/100 KBr solution is necessary to produce a permanent cloud of silver bromide. Basing your procedure on the results obtained, carry out a second series of experiments, arranging the volumes of each liquid taken so that the total volume of solution (including the added KBr solution) is always approximately 100 ml. This second series should cover at least a threefold ranee of ammonia con.~ wntratim. For thp s w m d cerice, plot thr. lug tlw volume of I
