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Understanding Acid-Base Titrations using Microsoft Excel

William Thomas, Colby-Sawyer College
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project

Summary

pH is a concept that gives many students difficulty after even several exposures from different perspectives in different classes. In this exercise, students will gain experience with acid-base titrations. Using an Excel spread sheet with embedded graphs, students will be able to carry out model titrations using "virtual" solutions of chosen molarity, visualizing the outcomes as they unfold. The titration exercises will be introduced and described through text that will help students understand the relationships playing out before their eyes.

Learning Goals

The goal is to help students understand how a change in pH (a log function) is dependent upon the change in hydronium concentraton (a simple linear conversion) that occurs during a typical acid-base titration.

Context for Use

This exercise is designed to be used in preparation for an early laboratory in our introductory cellular and molecular biology class. One discrete task in this laboratory exercise is the preparation of solutions to be used in later laboratory exercises. At a more basic level, students learn concepts and skills basic to the work that they will do throughout the remainder of the semester. A real comfort with the skills and concepts introduced in this simple exercise are quite important to students' success in the lab, and thus in the course.

In the past, students have had considerable difficulty with this exercise, often taking much of the laboratory period just to make the required solutions. The slowness with which they performed these simple tasks put them at risk to complete the more substantial tasks in the exercise. The teaching module "How sweet is your tea?" (available at the SERC site) was prepared to help the students formulate the solutions. This module is designed to help students understand what happens as they bring those solutions to the correct pH.

Typically these teaching modules would be assigned as homework to be completed by the students prior to the actual laboratory exercise. The modules would be appropriate for students in an introductory chemistry class, as well.

Description and Teaching Materials

This is a self-contained PowerPoint teaching module. All the required tools and information reside in the Excel files around which the PowerPoint module is constructed. They are used to define the characteristics of the virtual solutions to be use (e.g., volumes and concentrations), to enter the data that constitute the virtual titrations, and to visualize graphically the results in real time. The PowerPoint itself serves as a framework within which to introduce the concepts and to guide the learner step by step through the learning process, accessing the appropriate Excel files as necessary. By posing critical questions at appropriate times, it also provides the means to help students think about the results that they generate.

Because this PowerPoint teaching module is essentially a computer simulation, it can stand alone, allowing students to perform multiple virtual titrations without having to be in the laboratory or to manipulate actual solutions. However, the module finds it greatest utility as a prelab exercise that prepares students to carry out real titrations using prepared solutions.

Teaching Notes and Tips

Although this PowerPoint learning module does contain questions to which students must respond directly, it is designed neither as a writing exercise in its own right nor as the core of another writing exercise. It is assigned in the context of a multi-faceted laboratory exercise for which each student must ultimately write a comprehensive, coherent report. An understanding of pH is essential to a clear exposition of several elements of the laboratory exercise, so this learning module assumes an important role in helping students to write that report.

As stated above, this PowerPoint module is designed to be a stand-alone learning environment that a student (or group of students) can use on his/her (their) own. In practice, however, students exposed to this mode of learning for the first time may have some difficulty mastering the content as they learn their way through the PowerPoint format. Some explanation of what to expect prior to use of the module would be helpful. Even more helpful might be to use the module as a prelab exercise during which an instructor or teaching assistant would be present to guide students through any steps that might prove particularly challenging for them.

Assessment

Assessment can be evaluated in two ways. First and most directly, evaluation is based upon successful completion of the module, as exemplified by correct construction of the Excel spreadsheets and associated graphs. Secondly, evaluation should be based upon students' ability to apply the principles learned to new settings. For example, can they correctly explain what to expect were they to titrate a given solution with an acid of chosen molarity?

References and Resources

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