The DETERMINATION of CHEMICAL
EQUIVALENTS
&y the
EUDIOMETERMETHOD
AN EXPERZHENT FOR GENERAL CREHZSTRY STUDENTS
Carletan College and University of Idaho
T
HE RECENT tendency has been to increase the as shown by a careful study of the results obtained by amount of quantitative work in the general chem- students. We have, therefore, carried on over several istry course, thus calling for more refined appa- years a careful supervision of the work of the students ratus, and demanding much more careful manipulation to ascertain not only their limitations hut that of the by the student. Replacing the usual equipment with method as well. accurate balances and other pieces of apparatus essenDESCRIPTION OF THE EXPERIMENT tial to auantitative work is emensive. and with the large classes taking elementary chemistry the initial ~h~ type of apparatus* that was finally developed is cost and upkeep seem almost prohibitive. represented in the accompanyThe degree of accuracy required in order to make ing figure, This consists of an these experiments justified requires a manipulative especially designed small Er. skill possessed by few beginning students. To acquire lenmeyer flask of about 20-ml, this to a suFicient degree may mean repeating the ex- capacity with a side-tube (made periment several times. A satisfactory quantitative . from 7-m, tubing) attached experiment will he one which can be carried out in a at bottom,+ The neck of reasonably short time, thus permitting some repetition the flask is of such a size (about without requiring extra time of the student outside of mm,) that a 100-ml, eudiomelaboratory hours. This is of importance, in considera- ter will slip over it, The tion of the closely packed programs of most general flask is placed in a 250-ml, or . chemistry laboratories, if the question as to the relative 400-ml, and a thistle .. values of this or some other exercise is not to arise. tube is attached to the sideAnother question in connection with quantitative tube of the small flask by a experiments deals with the extent of the calculations piece of mbher carryand interpretations of the data to he made by the ing a pinchcock, student. It is well known by all tea&ers of general Samples of magnesium should chemistry that students have great difficulties with weigh about the same number these simple numerical calculations. Despi& the fact of milligrams as the number that the calculations are time-consuming, and require of millimeters of hydrogen gas considerable work of the instructor to find the students' desired, while the weight of mistakes, show them how to set up their equations, zinc should be about 2,5 times etc., we feel it is time most valuably spent. as great for the same volume The following experiment for the determination of of gas (uncorrected). Be- chemical equivalents has been in use a t Carleton Col- fore placing the metal in the lege for the past ten years, and it has occurred to us that Erlenmeyer flask see that the possibly others might like to utilize it. We feel it is a apparatus is completely filled thoroughly satisfactory quantitative experiment since ~ t distilled h water and that EPUIYAthe chemical principles illustrated by it are importaut, air has been driven out the calculations and interpretation are not too difficult, of the rubber connections. the laboratory manipulation is short enough to permit H, the eudiometer filled with water and inverted repetition if necessary, and finally the equipment is not * A more detailed description of the apparatus and the pros' as to be prohibitive' As this 's probably cedure will be found in "Labaratory manual in general one of the most common quantitative experiments istry" (to Z C C O ~ ~ ~ ~ ~ L I N C O L N A N D B"General A N R S , chemistry"), by A. T. LINCOLN AND G. B. BANKS, Prentice-Hall, Inc., New employed in general chemistry, we have thought it would he of interest to present, in conjunction with the Y ~ " , ~ ~ ~ s p e Erlenmeyer cial flasks were made for by the description of our method, the limitations of the method Central Scientific CO., Chicago, 111.
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over the beaker. Now wrap the weighed piece of metal in a piece of 60-mesh copper wire gauze about one inch square, securing the ends so that none of the metal can escape (small bits of the magnesium especially tend to float out). Moisten with water to remove all the air from the gauze and replace it with water. Now introduce the metal wrapped in gauze into the small flask, and place the inverted eudiometer over its neck. Place about 15 ml. of commercial hydrochloric acid in the thistle tube. Open the pinchcock and run in slowly a little acid to start the reaction. Do not introduce the acid too fast as the hydrogen may be evolved so rapidly that i t cannot get up the eudiometer and may come out underneath and be lost. After the reaction has nearly stopped add more acid, being particularly careful not to run out all the liquid from the thistle tube thus introducing air into the hydrogen. It is well to fill the thistle tube with water to prevent this. After the reaction is complete transfer the eudiometer tube of hydrogen to a tall cylinder of distilled water. Clamp the tube so that the surface of the liquid inside is a t the same level as that outside. After ten to fifteen minutes read the gas column as accurately as possible, employing the under surface of the meniscus. Also read the temperature of the air and take the barometer reading. The student is instructed to record all these readings in permanent form as the data of the experiment to be used i n the subsequent calculations. CALCULATIONS
To show the quantitative relationships it is necessary that all the data be as accurate as possible. Because of the inexperience of the students, we give them samples carefully weighed by the instructor on the best quantitative balances. It is the duty of the student to calculate from the volume of hydrogen obtained the weight of the metal sampleissued to him. He takes this value to the instructor, gets the actual value, and then calculates the percentage erfor. He is asked to calculate the following values from his data, remembering that the gas is collected over water and that a correction must be made for the vapor pressure of water: (1) the weight in grams of hydrogen gas liberated, (2) the weight of the sample necessary to yield this weight of hydrogen, (3) the percentage error as indicated by comparison of this value with the weight recorded by the instructor, (4) the number of grams of the metal sample equivalent to eight grams of oxygen, (5) the number of grams of hydrogen which would have been liberated if a gram atomic weight of the sample had been used. In some laboratories the instructor may desire to have the student weigh his own sample, in which case the necessary changes in the requirements are obvious. We feel that unless the balances available for the student are of the best analytical quality the student must be furnished with weighed samples.
DISCUSSION OF STUDENT RESULTS
In the accompanying table will be found some of the actual results obtained by the students. STUDENT RESULTS
Meld
Weigh: of Meld Colc. 6y Sludcn: By Boloncc
Diffmncc
Pcr~anrogc
Error
These data suffice to show the character of the results obtainable. From a study of the percentage errors of the student results we have found that approximately two-thirds of the students will have errors smaller than 0.2570. Also there will be extremely few students with errors over 0.50%. The limit of error to be required of the student might well be set a t 0.50%. Of course the results from the zinc samples tend to be more accurate, with a larger number of students having an error near 0.10%. Most of the results in error by more than 0.50% were obtained on the student's first trial, and a second trial will almost invariably reduce his errors to less than 0.50%. MORE PROBABLE SOURCES OF ERROR
As previously emphasized,'the student's chief d s culty lies in the arithmetical calculations. I t is rare not to find some error in the student's calculations. Our first step is to always check carefully his calculations. Very few students can use a slide.de or logarithms, so that the laborious mechanical process of calculation with its attendant inaccgracies is usually employed. Students, of course, do not know how many figures to retain or drop in their calculations to insure accuracy to tenths of a per cent. Since their values usually involved the fourth and fifth decimal places these have been utilized to compare with the set of exact weighings that were made of our known samples, and further t o ascertain to what accuracy the samples should be weighed in order to justify a certain percentage accuracy demanded of the student. Assuming the usual amount of magnesium taken (0.08 g.) and an average amount of gas (80 ml.), a variation of approximately lo in reading the room temperature would represent an error of approximately 0.35%. A variation of +2% is possible if the measnr-
ing cylinders are placed near windows in the winter season, or if uncalibrated thermometers are used. If we assume that an error of 0.2 ml. in the volume of approximately 80 ml. is made, this would represent an error of about 0.25%. As many of the eudiometers are divided to '/& ml., the students find it easy to make an error in reading the volume of the gas. If these two data are in error at the same time it is then possible to have an error of about 0.9% or 0.4%, depending upon the particular combinations of the factors controlling the source of errors introduced. Then there is the error in the weight of the sample. A variation of 0.5 mg. in the weight of zinc and 0.2 mg. in the weight of magnesium represents an error of approximately 0.257&. So when the student himself weighs the sample to the nearest milligram, as many directions state, and on the usual type of balance utilized in laboratory work in general chemistry, with
his inexperience with such delicate manipulation, he cannot be held to a very high degree of accuracy. Now the question arises, if such exercises are t o emphasize exact quantitative relationships in chemistry,should they not be of a sufficiently high degree of accuracy to demonstrate to the student that these materials are related in a demonstrable quantitative manner? This type of exercise is among the first that he meets emphasizing the quantitatively exact nature of the science of chemistry. I t is therefore important to put some evidence before him in such a way as to impress him with the exactness of the verification of the statements that have been made, and to begin the emphasis of what we understand by the scientific method. I t is for these, among other reasons, that we prefer to give the students accurately weighed samples for the determination of the chemical equivalents of elements.
