LABORATORY UNKNOWNS IN GENERAL CHEMISTRY

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  • LABORATORY UNKNOWNS IN CHEMISTRY 919

    LABORATORY UNKNOWNS IN GENERAL CHEMISTRY.BY LYMAN J. WOOD AND SEWARD E. OWEN,

    St. Louis University, St. LOUIS) Afo.The usual course in beginning chemistry requires the student

    to perform experiments, the results of which are for the most partknown from the beginning. Such experiments, which we mightcall descriptive in nature, while essential in many cases, do notin general stimulate the curiosity of the student. Because of alack of interest, the student frequently makes a note book recordthat has no particular relation to his experimental, work, thematerial in the note book, of course, describing the results whichhe knows he is expected to get. In some cases probably no ex-perimental work is done at all.

    In order to avoid these and other difficulties described by oneof the authors in a recent paper (Lyman J. Wood, Jour. Chem.Ed. Vol. 3, p. 1313, 1926), a number of unknowns have been de-vised to take the place of or accompany a considerable portionof the usual descriptive experiments in freshman chemistry.These unknowns have been found to stimulate interest and alsoto afford opportunity of adapting the laboratory requirementsto the ability of the individual student since in many cases someof the unknowns can be made more difficult than others.A few years ago the assignment of more difficult work to some

    students than to others would have been considered as unfairand might be so considered by some teachers even now. How-ever, it is believed by the authors, as well as others, that a certainminimum-amount of work should be required of all students andthat above this minimum not more work, but a higher type ofwork should be required of the students of greater ability.The United States government recognizes difference in physical

    size when issuing army uniforms. The quartermaster is, accord-ingly, supplied with three types of uniforms, i. e., "longs,""shorts" and "stouts." There are about eight different sizes ofuniform to each type, and therefore approximately twenty-fourchances of fitting a uniform on a man. It would be foolish indeedto expect to fit the same sized uniform, or even the same type ofuniform, on every man entering the army. Possibly the mentalcalibre of students in college may be even more varied than thephysical sizes of men in the army. It would seem almost equallyas foolish to expect to fit a uniform inflexible chemistry courseon all students as to try to fit the same uniform on all men

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    entering the army, except as circumstances make it unavoidable.Some effort has been made to recognize this difference in

    mental calibres in developing the series of laboratory unknownsin general chemistry, described below. It has been found possible.in many cases to devise a series of unknowns for a given ex-periment which require widely varying student abilities. In thisway independent work is required of each student and also thecalibre of work required may be made, at least to some extentproportionate to the ability of the student.

    UNKNOWNS.Some of the unknowns used in our course are listed and briefly

    described below. Doubtless many others might be added so thatthe needs of any teacher of chemistry could be met.

    Quantitative Unknowns. (1) "Determination of grams percc of an unknown wooden block." The purpose of this experi-ment is to become familiar with the metric system of measure-ments. The block is carefully measured and weighed and theresults are reported and the instructor checks the figures by atable of known dimensions. Although this determination isvery simple it has not been found to be without value to mostbeginning students.

    (2) "The determination of grams per cc of an unknownmetal." Different lengths of rods and tubes of such metals asiron, lead, copper, zinc, tin, and nickel are issued. The studentweighs the metal sample to the nearest centigram and obtainsthe volume by displacement in carbon tetrachloride in an alkaliburette. His reported findings are checked by the instructoras in the previous experiment.

    (3) "The determination of per cent of oxygen in an unknown."The student knows the unknown to be either potassium chlorateor potassium perchlorate and is asked to determine the per centof oxygen and incidentally to determine whether he has chlorateor perchlorate. The unknowns are issued ready mixed withpowered manganese dioxide in the ratio of three parts of un-known to one part of maganese dioxide.

    (4) "Determination of number of grams of sulphur to unitewith one gram of an unknown metal." The student knows hisunknowntobe one of two metals as nickel or copper, iron or nickel,nickel or lead, etc. The metal is issued in the form of a powdermixed with an equal quantity of sulphur. A weighed amountof the sample is heated to complete the reaction and the excess

  • LABORATORY UNKNOWNS IN CHEMISTRY 921

    of sulphur is distilled off. The weight of sulphur per one gramof metal is determined and incidentally the formula of the sul-phide is calculated. Pairs of metals whose atomic weights arenearest to each other are reserved for the better students.

    (5) "Determination of the number of atoms of chlorine tounite with one atom of a metal sample." The student obtainsa sample of metal and dissolves it in aqua regia. The solution isthen evaporated to dryness and the weight of chloride formed isdetermined. Cadmium has been found to be an admirable metalfor this determination. Table one shows the results obtainedby a small class under carefully controlled conditions during thepast summer.

    TABLE ONE.Showing the number of atoms of chlorine for one atom of cadmium

    obtained by several beginning students.Students laboratory Atoms of chlorine per

    number atom of cadmuim

    1.912.051.991.911.902.231.99

    The average figure for the number of atoms of chlorine for oneatom of cadmium is well within the limit of experimental error.

    (6) "Determination of the normality of an unknown acidsolution." Each student makes up an alkali solution whosestrength is approximately two normal and then standardizesthis solution by adding an excess of hydrochloric acid, evaporat-ing the solution and weighing the amount of salt formed. Theoriginal alkali solution is then diluted and titrated against theunknown acid. Individual unknowns are readily made up bymixing measured amounts of five normal acid and water fromtwo burettes.

    (7) "Quantitative determination of the amount of oxygen ina gaseous mixture by the use of alkaline pyrogallate." The pro-cedure for this experiment varies from the usual only in thatthe student obtains from the store room an unknown mixtureof nitrogen and oxygen in a test tube over water. The unknownmixture may be ordinary air, air enriched with oxygen or airenriched with nitrogen.

    Qualitative unknowns. (8) "The properties of a pure sub-stance." An unknown is provided which is either a pure sub-

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    stance or a mixture. The student must perform distillation,crystallization, solubility and evaporation tests and reportwhether he has a pure substance or a mixture. Some of thematerials used for this experiment are listed in table two.

    TABLE Two

    Showing composition of unknowns which students must prove to be amixture or a pure substance.

    IngredientsKey letters

    (given to students)WaterChloroformSaltSulphurWater and saltSulphur and saltWater and alcoholLead chloride with potassium chloride

    anolgrmnhewiskxy

    (9) "Determination of the conductivity of solutions." Thestudent is supplied with several solutions or substances to beput into solution. He places these successively in a beaker and.immerses in the solution two electrodes arranged in series withan electric lamp on the regular lighting circuit. If the lamplights there is conductance. Any carelessness on the part of thestudent in washing beakers or electrodes between tests is easilydetected when the unknown is reported.

    (10) "The chemical tests for the halides." Three unknownsare issued to each student in numbered test tubes as is illustratedin table three. The use of numbered test tubes makes the pos-sible number of different unknowns quite large. If M denotesthe number of permutations of n things taken p at a time it isknown that

    M=n(n-l) (n-2) (n-3)................(n-p+l)and in this case M equals 24 but if combinations including morethan one similiar unknown are used the possible number of dif-ferent unknowns mounts to 64.No one tube contains more than one halogen except in the

    case of some of the better students. These unknowns follow aseries of preliminary work on the halides. In our laboratory thenon-halogen mixtures usually contain phosphates or carbonatesor both. With acid the carbonate liberates a gas and with silvernitrate the phosphate gives a yellow precipitate similar to thehalides; this precipitate, however, is soluble in nitric acid. Suchunknowns tend to keep the student constantly on the alert.

  • LABORATORY UNKNOWNS IN CHEMISTRY 923

    TABLE THREE

    Showing method of issuing halogen unknowns

    StudenVs Lab.Number

    1234567

    Tube 1

    C1BrBrII0101

    Tube 2

    Br01I01BrBrX1

    Tube 3

    II01Br01X1Br

    SEMI-UNKNOWNS.As a means of giving individuality to the work, at the

    beginning of the year, a laboratory number is assigned toeach student. In certain experiments, as for example in thework on Boyle^s law the student uses a volume of air which isrelated to his own laboratory number. Each student thusobtains a set of data that is not duplicated by others and to acertain extent, his problem is individual. It has been found thatthe laboratory number gives a new interest to such experimentsand that the psychological effect produced by its use is excellent.