J. R. Masaguer, M. Victoria Coto, and J. 5. Casas University of Sontiago de Compostela, Spain
An Experiment for Introductory College Chemistry How to establish a chemistry equation
Determination of the stoichiometry of a chemistry process by using the Continuous Variations method* is almost an everyday topic in the laboratory. Until now, the experiments available, on an elementary basis, always required lengthy and tedious permutations of weight measurements or thermal variations making them of little practical use when the groups of students who have to do these experiments are numerous. This paper proposes an easy experimental process using the Continuous Variations method, an experiment which can be accomplished by the student in a reasonable amount of time. The stoichiometry of the reaction between potassium chromate and barium chloride in an aqueous state is determined by using the height of the precipitate which forms when mixing different amounts of both solutions. Experimental Procedure Pour into graduated test tubes the volumes of barium chloride, 0.5 M,which are indicated in Table 1 and then warm them for a few minutes in a hat water bath. To the hot solutions, add from a buret the corresponding quantities of potassium chromate 0.5 M indicated in the table and shake the tubes. A precipitate forms and should be left to settle for approximately half an hour. Measure the height of the precipitate by using the graduations on the tube. Use these heights as ordinate values and the number of moles of the two solutions as abscissa values to plot a graph of the point of inflection of the results. Establish the abscissa which corresponds to that point. The figure represents the data obtained by working according to the above procedure.
Discussion It is worth remembering that because the precipitate settles by gravity, there may be anomalies in its height in the tube, mainly because of variables which are difficult to control. Nevertheless, the consideration of all the tubes a s a whole always gives an accurate experimental conclusion. Since the experimental data which the student bas to measure is very accessible (he only has to bear in mind the graduations within the tube). . . this exneriment can be performed by courses in elementary chemistry that have reached a hieh enough level to understand the Continuous Variation method.
Plot of experimental results.
Quantities of Reagents Used Run
BeCh imll
KxCrO. imll
This experiment has the additional advantage that when the yellow color of potassium chromate appears, those tubes in which the precipitation is not complete are left with a floating yellow residue, while in the remaining tubes the solution remains colorless. This point will help the student to establish his conclusions. All the materials needed for this exercise are in common use and easily available in every laboratory. ' S e e "Inre.-ligating Chemical S?.srems." CRA. Ed. N'rb3trr D i ~ ~ w ,.\lrCrcu-Hill n. Hook Company, Experrment y.
Volume 52,Number 6,June 1975 / 387
