PROFICIENCY of FIRST-YEAR STUDENTS in QUANTITATIVE

EXPERIMENTS BRUCE S. FARQUHAR WITH FRANCIS EARL RAY

The University of Cincinnati, Cincinnati, Ohio

S INCE Herbert Spencer published his famous essays on education, the battle has raged over

"What knowledge is of most worth." Without venturing into the general topic, we would submit that what is done must be well done if i t is to accomplish any of the advertised objectives. The progress of civilization has been marked by the steps taken to es- tablish standards of value. In the last two decades the "standard test" has been introduced into our educa- tional system as an aid in comparing the work in different institutions. While an examination into the merits and demerits of the standard test is beyond the scope of this brief note, i t will be obvious that a "stand- ard test" can give no information in regard to the pro- ficiency attained in laboratory technic. It was, there- fore, with the idea of encouraging the establishment of norms for laboratory performance that we undertook a detailed analysis of the achievements of students in our beginning laboratory course in chemistry.

EXPERIMENTAL DATA*

Comparison of grades is not very informative, for a grade is often a better indication of the metabolism of the grader than it is of the quality of work done by the gradee. We selected, rather, the actual figures obtained by undergraduates in quantitative experiments. The results we present, while limited in scope, are universally comparable. It is hoped that others will publish their data so that standards of achievement may be avail- able for all quantitative experiments.

Synthesis of Cuprous Sulfide. (Figure 1.) This is the usual experiment in which a weighed amount of copper is heated with an excess of sulfur. All weigh- i n g ~ were made with hom-pan balances. We have tried beam balances but the cost of these made i t necessary to assign several undergraduates to each balance. The attendant confusion and loss of time led us to abandon the practice. The results with horn-pan balances compare favorably with those obtained with beam balances. T h e limiting factor seems to be the manipulative skill of the student rather than the sensi- tivity of the balance.

Figure 1 shows the distribution curve. The percent- age of copper reported is plotted against the number of students repotimn this percenta~e and a smoothed

curve drawn. The theoretical percentage is 79.9. Of 160 students all but three came within 10% of the correct value, as shown by the solid vertical lines. The dotted lines show that 50% of the class came within the values 76-81%. The error, it will be observed, is much greater on the low side of the theoretical value, showing that the excess sulfur was not always com- pletely burned.

I I I 1

30

25 4 $0 +

"0 15 8 2 10

4.3 4.6 5.0 5.4 5.5 Molecules HxO reported.

FIGURE MOLECULES &O IN HYDRATED COPPER SULFATE

Water of Hydration of Copper Sulfate. (Figure 2.) In this experiment a weighed amount of copper sulfate is heated to constant weight. As there are five mole- cules of water in blue vitriol, any value below 4.6 or - above 5.4 would have no significance. Only five - -

'Complete details of each will be found in students in 160 failed to come within this limit. As perimentalchemistry," by F. E. RAY (J. B. Lippimtt Co.). Figure 2 shows, the majority came much closer than

this. Thirty-four, or 21%, failed to come within the limits 4.8-5.2, which represents a four per cent. error. The curve also shows that overheating of the sample is about as common as underheating.

35

30

25 % a < 20 ". b15 n

10 z 5

0.60 0.64 0.68 0.72 0.76 Normality reported.

FIGURE 3.-THE NORMALITY OR VINEGAR

The Normality of Vinegar. (Figure 3.) The alkali is standardized by titrating i t against standard (0.25 N ) acid. The vinegar is then titrated and its normality calculated. Allowing a spread of from 0.64 to 0.72, which is about 6%, we tind that only two students ex- ceeded this ranpe, while 13 fell below. Ninetyper cent. of the class G g h t thus be considered as doing ac- ceptable work. The maximum of the curve is to the right of the correct normality. This is somewhat re- markable in view of the fact that the greatest devia- tion is to the left. One might infer that the careful student tended to get slightly high results while the careless person reported results much too low.

Molecular Weight of Sucrose. (Figure 4.) A weighed amount of sugar is mixed with cracked ice and the

temperature is recorded. The solution is decanted and the weight of sugar and water obtained. The tem- perature lowering is about five degrees. The error in taking the temperature alone is close to 10%. It is, therefore, somewhat surprising that over 80% of the students came within 10% of the correct value, 342. Of a class of 160, 93% came within 15% and 98% came within 20% of the correct value. The chart (Figure 4) shows that the error was mostly on the high side. Failure to reach the true equilibrium temperature was, no doubt, the cause of this. Values between 323 and 393 were considered acceptable. This covered 85% of the class.

Equivalent Weight of Aluminum. (Figure 5.) This determination is made by measuring the volume of hydrogen liberated by dissolving a weighed amount of aluminum wire in acid. Loss of gas should be the source of greatest error. It is, therefore, quite re- markable that Figure 5 shows that most of the values were on the low side. The only explanation that can be advanced a t present is that the heat of solution of the metal in acid caused a portion of the gas to occupy a larger volume than it normally would. This hypothesis will be tested out in subsequent classes. If an error of 10% is assumed to represent the limits of creditable work, we find that 14 of 154 students reported values below 8.1 for the equivalent weight. Eleven students reported values above 9.9. This means that 16% of the class failed to come within 10% of the correct value. This percentage, singularly enough, coincides almost ex- actly with the percentage of undergraduates failing the course.

It is the authors' belief that if many similar data could be collected from colleges all over the country a valuable contribution would be made to chemical education.