comments, no files. Students run programs using NUDRAW on any TEKTRONIX graphics terminal. Execution requires 7 K 16-hit words on a PDP 11/70 minicomputer. Documen- tation consists of a table of the available organic moieties, the subroutines (61 in all) that must be called to draw them and a short auide, with examples, of how to apply NUDRAW in

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programming. ('opies of the listin): and d~rcumentntion are a\vtilat~le free from the author nL the address given above.

A Computer Program for Studying Stoichiometry Problems

Cheryl J. Vaughn, Robert Morris and Toby F. Block University of Wisconsin-Stevens Point Stevens Point, WZ 54481

The ahilitv to solve stoichiometrv nrohlems is one of the most important skills a student ingeneral chemistry must master. We have developed an interactive promam (MOLES) that is designed to he$ students learn to work such proh- lems.

The program generates, at random, questions such as "How many grams of carbon are there in 125 grams of glucose, CeH,.,Oc?" or "How manv hvdroeen atoms are there in 1.56 .- moles of formaldehyde, d ~ x ~ ? " ? h e student is required to s u n ~ l v both the units and the numerical value of the answer (aiiide rule or pocket calculator should accompany the stu- dent to the terminal). Incorrect resDonses are checked for common errors, such as ignoring a subscript or Avogadro's numher. If such an error is found, the student is informed n h ~ ~ u t it and given a second rhanre to find thecorrect answer. If the cornpuler cannot identify the source of trouble, the computer supplies the correct answer and shows how it can he obtained by the factor label method.

If the student cannot find the proper units or has no idea of how to work the problem, s h e can request help. The pro- gram responds by giving hints useful in obtaining the desired solution. If several requests for help are made, the computer once again supplies the correct answer and appropriate fac- tor-label set-up.

Program MOLES was written in Interactive (non-ANSI- standard) FORTRAN. It contains 470 statements and 104 comments. Execution requires 26K 48-hit words on a Bur- roughsd700. The prog&has been run via Decwriter, hut any terminal coupled to a sufficiently large computer would do. Documentation includes listing and several sample executions; students are given instructions via the terminal at execution time. No card decks or magnetic tape copies can he supplied. Because some of the format statements are specific to the Burrouehs svstem. it mav he necessarv to revise the Droeram slightly'oef&e executing i t on another computer. ~ b n - i t a n - dard statements will be indicated on the listing. Listings and sample executions may he obtained, free of charge, by writing o: Dr. Toby F. Block, Department of Chemistry, University

of Wisconsin-Stevens Point, Stevens Point, WI 54481.

A Computer Program to Develop Laboratory Schedules

G. R. Hertel University of Central Florida Orlando, FL 32816

The computer prugram described here is an aid to the in- structor who ii faced with rhe uroblem uf makine UD labora- tory schedules in which students, working in pair<&e rotated through a numher of required experiments during the term and, ideally, are paired differently each week. I t is a situation commonly encountered in physical chemistry laboratory where limitations of equipment generaily force some kind of rotation scheme.

A paper by M. T. Marron (1) describes methods for devel- oping such schedules for certain comhinations of st~rlents and

experiments and gives some suggestions for working out schedules for other comhinations. While this publication is very useful and helpful, the task of preparing schedules for comhinations not specifically covered can still he very time- consuming.

A computer program has been written to develop schedules in which N S students, assigned in N P pairs to NE experi- ments, are rotated through NW weeks with the restriction that no two students are paired together more than once. Using the letters of the alphabet to represent the individual students, a matrix is constructed in khich unique pairs of letters are assembled into NE columns in such a wav that each letter appears once, and only once, in each column and each ,nw . .. .. .

If the numher of student pairs exreeds the numher clf ex- prrlments, the NI' X NP matrlx is rreated and thm tr~mrsted to an .VE X A'P rertaneular matrix. Students haw m e or more

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weeks off under this scieme. The schedule requires N P weeks to complete.

If the numher of student pairs is less than the numher of experiments, the NE X NE matrix is produced with blanks appearing throughout indicating that one or more experiments sit idle each week. The schedule takes NE weeks to com- plete.

If an odd numher of students is enrolled, the program will construct a schedule in which one student (a different one each week) must work alone.

As Marron (1) points out, cases in which N P = N E can he solved for all cases except NE = 2 and NE = 6 by superposing two orthogonal Latin squares, producing a Gra&ol~atin square. Since this orthogonal Latin squares approach is more restrirtive than nectssnry in this scheduling application, it is not strictly ti,llowed in constructinr the marrices. The pro- cedure that is used will produce a 6~ 6 matrix with unique pairings throughout. In fact, in its present form, the program will establish a matrix meeting the original criteria for all cases in which N P 2 NE (NE > 2), for all cases in which N P = NE - 1 (NP > 4), and for most cases in which N P = NE - 2 (NP > 4).

The program is written in BASIC, specifically, Level I1 BASIC for the TRS-M Microcomputer (2). I t should operate with any BASIC interpreter capable of handling string com- parisons and two-dimensional arrays. The maximum matrix that can he handled is limited by available memory. The TRS-80 Level I1 Microcomputer with 16K RAM memory can handle matrices up to and including size 15 X 15 (maximum of 30 students) successfullv. To exceed N S = 31 certain nro- gram lines must he modified to accomodate a more compatible section of the ASCII code for the lower case letters and since the TRS-80 does not display lower case, the display would get confusing). Also, to print out large matrices on the TRS-80 (NE > 15) some modification of print statements would he necessarv. The program itself occupies an~roximatelv 8.5 . . kbytes of memory. '

A description and program listing-printed by a Radio Shack Quick Printer 11-and a TRS-80-compatible cassette tape are available from the author. Send cheek or money order made out to G. R. Hertel, Department of Chemistry, Uni- versity of Central Florida, P.O. Box 25000, Orlando, Florida 32816 ($3.00 for the program listing; $10.00 for the cassette tape).

The Periodic Chart at the Tip of Your Fingers

Lucy T. Pryde Southwestern College Chula Vista, CA 92010

Pocket programmable calculators are hemming increasingly popular among students, even if they have access to lareer more complex computers. There is a fascination in havingso

Volume 57. Number 4. A ~ r i l 1980 1 251