George R. Wiger and Ulrich de la Camp California State University Dominguez Hills, California90747
Conjugate Acid-Base Mixtures in the General Chemistry Laboratory
I
A comprehensive buffer experiment
Proton acids and bases occupy a position of central importance in science majors' general chemistry courses. Their importance is not only emphasized in the lecture portion, hut a good deal of laboratory time is often devoted to studying acid-base systems. Generally the laboratory, regardless of the manual used. consista of the same exoeriments: a) measurine the pH of a variety of substances (apple juice, coffee, etc.); bi determination of the GEW of an unknown acid by titration; and c) determination of an acid K.. Occasionally this is embellished somewhat by doing a complete titration curve, but by and large there is no deviation from the three basic exercises. Now, i t should be stated that these are good, solid, and worthwhile endeavors and should be included in general chemistry lab. There is considerable chemistry here, not only in reactions and equilibrium, but also in some basic techniques such as use of pH meters, methods of proper titration, and of pipetting. It is-our opinion, however, chatan area of acid-base chemistry which is of central importance, namely buffer systems, is largely ignored in the general chemistry laboratory. Indeed, a truly comprehensive experiment in buffer chemistry is not to be found in any general chemistry laboratory manual of which we are aware. We wish to report here an experiment which we feel eives the student a eood demonstration of the behavior of a&-base buffers. ~deafiy,it will allow the student, using experimental data which he or she has gathered, to develop the Henderson-Hasselbach relationship and also understand. in a auantitative fashion. the behavior of buffers toward the twomost common stresses which they are likely to exoerience: a ) dilution b) addition of an acid or a base. ~ e f o r we e discuss details of the actual experiment several other statements should he made. First, this is not an inquiry based exercise. It is of necessity very structured and very exact directions are given. However, in order to make it more effective, the instructor presents no discussion, either pre- or post-lab and answers no questions. Hence, although the student is told exactly how &perform the measurements, it is up to the student to properly follow the instructions and interpret the resulta within the eiven euidelines. Second. the exneriment assumes the student understands the basics of proton acids and bases, pH, and can perform general ionic equilibria calculations. However, the experiment is carried out before any mention has been made in lecture of buffers and the term. "buffer," is avoided throughout. T h e experiment is entitled "Coniueate AcidIBase Mixtures" and that is the phrase utilized th;oughout.
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The Experiment Theexperiment itself i r divided into rhrer parts: I ) Mixing Iinticm 2) Dilutim31Etfett uf AddingaStrung Arid i w Hnw. Each part w ~ l l he dir~ursedindivduaily i w l w . Apparatus and Supplies The experiment requires the following equipment and chemicals per student, in addition to those generally available
Mixing Ratios In this section and the followingtwo, the student shares burets with a partner. Thus, each pair has one buret with 0.5Msodium acetate, one with 0.5 M acetic acid, one with 0.1 M HCI and one with 0.1 M NaOH. This makes the most efficientuse of materials. Although they share the burets, each does all of his or her work alone. The basic procedure consists of combining acid and base in a variety of volume ratios. dilutine the mixtures with distilled water and measurine .. DH values. Volume mtils ( h e wid) I 1, 3 1, 11111.1.5 and ltlll are ~mpl$.yed. The students nre then told ttr prepare the f
