Mass Is Better than Light: Students' Perception of Using Spectrophotometry and Gravimetric Analysis to Determine the Formula of a Hydrate in the General Chemistry Lab

A key part of the practice of chemistry is the analysis of chemical composition, including through gravimetric analysis and spectrophotometry. However, the complexity of doing multiple calculations to obtain analytical evidence, such as that required to determine an empirical formula, presents a challenge if such analytical methods are to be understood by students and if they support meaningful learning about other chemical concepts and methods. In this study, we investigate student use of spectrophotometry and gravimetric analysis to determine the number of water molecules in hydrates of copper(II) salts, a method previously described by Barlag and Nyasulu. Using phenomenography to analyze students' reports through the lens of meaningful learning, we identified four distinct perceptions and, within them, information on how students make sense of the complex analytical steps involved in the experiment. We identify how meaningful learning is present where students recognized that spectrophotometry was based on light-matter interactions (cognitive), was faster and more accurate (psychomotor), and allowed students to express confidence in the process and their results (affective). However, it is also the case that meaningful learning was compromised where students had trouble conceptualizing spectrophotometry, saw it as a set of disconnected steps, and saw absorbance as a computer-generated value and not a property of the solution. This led to the perception that gravimetric analysis provided a more direct and understandable technique. We discuss the implications of these findings for chemistry education research (CER) and curriculum development in the undergraduate teaching lab.