The paper presents the design and evaluation of a voluntary online introductory stoichiometry (VOIS) course aimed at facilitating the transition from secondary to higher education. The course utilized simple analogies and adaptive feedback through a formative scaffolding assessment. The study assessed the effectiveness of the VOIS course through pre- and post-knowledge tests, analysis of students' performance in general chemistry, and course evaluation surveys conducted at a Latin American University between 2019 and 2021. A total of 3,995 first-year STEM students enrolled in the course voluntarily, and 358 students successfully completed it. The results showed a statistically significant improvement in stoichiometry-related knowledge, with the pre-post test scores increasing from 4.61 to 6.55 out of 10. The matched sample analysis, which only included students with 100% participation, demonstrated a statistically significant improvement in stoichiometry and related knowledge from 5.31 to 6.61. Furthermore, an analysis comparing the performance of students who completed the VOIS course with those who didn't reveal that the former group outperformed the latter by an average of 10.6 points in the general chemistry course. This statistically significant difference exhibited a large effect size (d = 0.8). In addition, a matching technique was employed to construct a synthetic control group in order to reduce bias in the quasi-experimental design. A successful propensity score analysis was conducted, controlling for variables such as gender, grade in high school, scores in the national test, and student ranking in their high school. The results of this analysis showed a statistically significant improvement of 8.6 points in the general chemistry performance for students who completed the VOIS course compared to those who did not enroll in the course. Furthermore, the feedback from 129 respondents indicated that 80% of the students either liked the VOIS course or liked it very much, with an overall satisfaction rating of 3.1 on a four-point scale. In conclusion, the VOIS course demonstrated positive outcomes in terms of enhanced stoichiometry knowledge, academic performance, and student satisfaction. These findings highlight the potential of online courses like VOIS in facilitating the transition to higher education.