Stoichiometry is a significant yet challenging topic in chemistry education. While extensive research has explored students’ conceptions, difficulties, and learning approaches, this study adopts a competency-based approach to introduce a new model defining three competency levels in stoichiometry. The stoichiometry competency level model (StoiCoLe model) offers a framework for evaluating students’ performance in algorithmic stoichiometry. To test the assumptions of the StoiCoLe model, a 40-item test was developed to measure and categorise student's competencies according to the model's levels. Using data from 289 students enrolled in an introductory chemistry course across three semesters, psychometric properties and model assumptions were analysed through Rasch-analysis and item processing times. The results indicate that there is sufficient psychometric reliability in the categorization of students according to the StoiCoLe model. However, both the item difficulty and the processing times are only partially consistent with the assumptions of the model and indicate an adaptation of the model. In line with prior studies, the majority of students exhibited lower competency levels. These findings are discussed in terms of how the competency-based approach can enhance relevant competencies and contribute to literature on chemistry education in stoichiometry.
