The Lewis acid-base model is key to identifying and explaining the formation and breaking of bonds in a large number of reaction mechanisms taught in the sophomore-level year-long organic chemistry course. Understanding the model is, thus, essential to success in organic chemistry coursework. Concept-inventories exist to identify misunderstandings and misconceptions of acid-base theories; open-ended problems, though, have been shown to provide a more nuanced and holistic understanding of how students use acid-base models to explain reactions. The time necessary to score such problems, however, limits their use, especially in large student enrollment courses. Given the efficacy of open-ended problems, there is occasion for the development of methods to efficiently and effectively analyze open-ended assessment responses. In this study, we establish the importance of assessing "use of the Lewis acid-base model to explain a chemical reaction" by determining the association of model use with summative examination performance. In addition, we generate and evaluate a binomial logistic regression model based on lexical analysis techniques for predicting Lewis acid-base model use in explanations of an acid-base proton-transfer reaction. Our work results in a predictive model that can be used to score the open-ended problem used in our study.