EVALUATING UNKNOWNS in QUALITATIVE ANALYSIS H. W. MARLOW Kansas State College of Agriculture and Applied Science, Manhattan, Kansas
A basis for scoring a student's efforts in qualitative analysis is suggested. This gives the student credit for puving ions, and also groups, absent as well as present.
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STUDY of bases for grading unknowns in qualitative analysis shows that many teachers grade upon the report of the number of cations and anions actually present. Apparently, no importance has been attached to the greater number of cations and anions in such a wstem that were necessarily proved absent. Since it iuvolves as much labor and time on the part of the student definitely to prove an ion absent as it does to prove one present, the evaluation
of those absent should be comparable to those present. For example, if a student has two cations and one anion present in his unknown solution and fails to report one of them, his grade based upon his actual findings and not including his write-up, is approximately 67%. This report leaves 19 cations and 14 anions to be proved absent, for which he receives no credit. With this injustice in mind, the author decided to see if a fair and comparable evaluation could be made on the basis of the number present and the number absent. In consultation with other members of the chemistry department here, who were teaching this work, a value was placed upon each unknown in our list. In order to make the system clear the facts as used in
our scheme of analysis can be applied to a problem. Five groups of cations and three of anions were recognized. Groups of cations I No. of members tested for in each 3 Groups of anions I No. of members tested for in each 9
I1
I11
IV
V
5
6 I11
3
4
I1
4
2
Groups V of the cations and I11 of the anions have no group tests and hence each member is evaluated separately and each must be tested for separately. The values assigned are based on these facts: when a group test is possible, a value of one is given for proving the group absent. If a group is present, no value is given for the group test, but value is given according to the number of ions tested for in that group. For example, Groups I, 11, 111, and IV of the cations and Groups I and I1 of the anions are given values of 3, 5, 6, 3, 9, and 4, respectively. Groups V of the cations and I11 of the anions have values of 4 and 2, respectively, since there is no group test and each member must be identified separately. With these values in mind, let us cite an example. Suppose an unknown contained HgIl and KAl(S04)2, we may evaluate in this way:
Aniorr Group I Group 11 Group 111
Condition Pres~ot Preeenf Present
Volue 9 4 2 15
Total value is 34.
If calcium is not reported and strontium is, the value of the cation would be 19 - 6 = 13. If carbonates and chlorides are not reported, the value of the anions would be 15 - 6 = 9, or a total credit of 22, or 22/34 = 65%. Under the older system there would be a total of 9 to be tested for and the marking would be 5/9 = 55%. x t h i r d example will suflice. The unknown contains AgN03and AgC2Ha02. C0li0"~
Condilion
Group I Grovp I1 Gmvp 111 Group IV Group V
Prerent Absent Absent Absent Absent
Anions
Coldilio"
Group I Group 11 Group 111
Absent Absent Present 4
Total value is 14.
If nitrate is not reported a value of 14 - 3 = 11, or 11/14 = 79%. The experience of previous years indicated that apAbsent Gromp I present Group I1 proximately 100 points represented a semester's work rer rent Group I11 for the average student. If more work is completed, Absent Group I V Present Group V extra credit is given. Compared with the older method, this system appears to have the following advantages: A nianr Condition . Volur Group I Present 9 1. It gives the student credit for proving the abGroup I1 Present 4 Group I11 Absent 2 sence of many ions, as well as proving the presence of a few. (2 tests) 15 2. I t is adaptable to any, scheme of qualitative T O ~ value ~ I of the unkoown is 32. analysis with any number of cations and anions, and For each ion present and reported,absent and for each with any number of groups, because it sets up proper absent and reported present, a discount of three is values for the groups and the group numbers. made from the grand total of the unknown. Suppose 3. At the discretion of the instructor it may be re3 = cast so as to give more credit for the more difficult K was not reported, the evaluation would be 32 29, or 29/32 = 94%. Under the older system, since tests of each group and its members, the values being there is a total of 3 cations and 2 anions (5 in all) to be commensurate with the labor and time involved. reported upon, the value would be (if K is not reported) 4. Finally, it enables the teacher to assign impar5 - 1 = 4,0r4/5 = 80%. tially to each student a set of unknowns that involves The write-up, which is given to the instructor a t the the same amount of work during the semester, and conclusion of the work on an unknown, shows the steps hence furnishes the teacher with a comparable basis for by which the student eliminated or proved present evaluating the work of each student. groups and the ions in any group. This system which is used for our second-semester Another example: Suppose the unknown is ZnC03, freshmen students in the General Science Division, has Bi(NOa)r, CaClp, and KCr(SOS2. been in operation each semester and summer school for seven years. It is giving satisfaction to both instructor Colias Condition Volur and students. Students are naturally curious about Gmvp I Abocnt 1 Group I1 present 5 the value attached to an unknown and the reason for Group 111 ~ r c ~ n t 6 such evaluation. An explanation by the instructor Group 1V present 3 Grmp V Present 4 invariably meets with hearty approval as i t seems fair to give credit for proving ions absent as well as present. co1ions
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