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The Mole

Limiting Reagents

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Creation of an online stoichiometry course that melds scenario based leaning with virtual labs and problem-solving tutors

David Yaron*, Gaea Leinhardt†, Karen Evans†, Jordi Cuadros‡, Michael Karabinos*, William McCue* and David H Dennis*

*Department of Chemistry, Carnegie Mellon University, Pittsburgh PA 15213
†Learning Research and Development Center, University of Pittsburgh, Pittsburgh PA 15213
‡Institut Químic de Sarrià, Via Augusta 390, 08017 Barcelona

This paper will discuss an online review course in stoichiometry aimed at students who are about to enter college chemistry and need a review of this important foundation material. The course uses the ChemCollective's virtual lab ( and the course delivery and problem solving tutor tools of Carnegie Mellon's Online Learning Initiative ( The course is set in the context of arsenic contamination in the groundwater of Bangladesh. This scenario highlights the utility of stoichiometry concepts in a real world problem and allows us to, as the course progresses, shift the theme to the challenges facing modern analytical chemistry. The course contains 15 modules ranging from the mole and molecular weight up through reaction stoichiometry, empirical formula and limiting reagents. Modules typically start with a video explaining the concepts followed by a few simple tutors that serve as interactive worked examples and then either a virtual lab or more extensive problem solving tutor. Our experiences with creating and evaluating this course will be discussed.


This paper discusses an online stoichiometry course created through collaboration among chemists, educational psychologists, and learning technologists at Carnegie Mellon University and the University of Pittsburgh. Our principal motivation was that stoichiometry is important base knowledge for introductory college chemistry and one that remains a barrier to success for many students throughout a one-year introductory chemistry sequence. An online course seemed a potentially useful way to give students an opportunity to learn this material before or during the early portion of the college course. In particular, Carnegie Mellon, as well as many other universities, provides only a brief review of this material in lecture and instead requires students to self-study and pass a mastery exam sometime during the first semester course. In 2005, we offered students who were planning to take introductory chemistry in the fall the option of taking the online course during the summer before arriving on campus.

Why build an online course when there is no shortage of textbooks and study guides on stoichiometry? Stoichiometry is hard because it uses an opaque and somewhat ambiguous notational system and because it relies on proportional reasoning. Both of these features, notational and mathematical, have been shown to be very challenging even for science majors. However, stoichiometry is not THAT hard. In fact we might consider the following thought experiment: Suppose we offered students a $50,000 award if, after using text books alone, they could pass a tough test on stoichiometry. It seems likely they would pass. But if the offer were only $5, would they also pass? This seems less likely. So while such text-based materials do seem sufficient for learning, the learning may be quite challenging and effortful. If that is true then the question is perhaps better posed as "Can we create an online course that lowers the effort required to learn and retain stoichiometry?"

In designing our course, we had two factors that made us hopeful. One factor was that our team combined expertise in chemistry and educational psychology. This would allow us to incorporate core principles of learning and develop a task analysis that included not only the components of the computational procedures, but also how this knowledge fits into the content and practice of the domain. The other factor was the use of technology to provide better, dynamic explanations and to give students practice that is both scaffolded and authentic. What remains less clear is the extent to which we were able to use the technology in service of learning as opposed to vice versa.

One of the first design decisions related to linearity versus modularity. The course is packaged as a linear sequence of topics. The linearity guarantees complete coverage and integration among procedures and concepts. However, linearity (with internal branching) means that the uses of stoichiometry are presented as ends in themselves. The alternative would be to introduce stoichiometry as needed in the service of other more significant chemistry topics. To the extent that stoichiometry is a toolkit that experts invoke while working on larger issues, it may be better to teach the material as needed in service of these more authentic pursuits. Such a just-in-time approach could instill more flexible and robust learning, such that students can better access the knowledge as needed in their future learning. We have, to some extent, created the materials in a way that the linear sequence may later be decomposed and the various components used in such a learn-as-needed approach. In addition, we have attempted to provide some of the benefits of learn-as-needed within the linear sequence by setting the learning in a real-world scenario (arsenic contamination of the ground water of Bangladesh) and by providing virtual labs that allow students to apply their knowledge in an environment that is more authentic than that of paper-and-pencil. The extent to which these structures help us meet our learning goals will be considered in more detail below.

The course can be accessed at The entire course can be viewed without creating a login, but creating a free account will allow progress to be recorded in the gradebook. The course admit code, requested after account creation, is 'stoichdemo'. Course sections, created on request to, allow the instructor to use the gradebook and assessment options to keep track of students' progress.

The course contains:

  • 29 flash presentations that set the context of the practice (arsenic contamination in the ground water of Bangladesh), give instruction in the course concepts, and provide worked examples. These presentations average about 3 1/2 minutes each.

  • 42 flash questions that provide hints to guide students through calculations (each provides 3 to 6 hints for each response, with the last being a bottom out hint that gives the answer). These play a role similar to that of worked examples in a textbook.

  • 6 virtual labs, including feedback that checks for common errors.

  • 3 parameterized tutors that help students learn the more complex stoichiometric calculations (empirical formula, limiting reagents, and mixture composition). The parameterization is sufficient in that the variation among instances is comparable to the variation among end-of-chapter problems in a textbook. The style is that of assistments, [1] which ask students for the response to a multistep problem and give step-by-step help only if needed.

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