Chemistry is very important at the tertiary level, but because of its abstraction, students feel hostility towards it. To improve this attitude, the focus seems to lie in a didactic approach that uses the construction and revision of mental modeling processes. The objective of the article is to lay out the case of a course presented with a scientific challenge in the stoichiometry unit that consisted in determining the number of molecules of a gas generated in a chemical reaction. The class was carried out through a process of co-construction of knowledge between learners and teachers and was organized in a didactic cycle of five stages. The students' mental models evolved through cycles of generation, evaluation, and modification, which was reflected in the high grades obtained by them in the final report, evidencing the positive effect of this methodology on learning.