Students' inaccurate ideas about what is represented by chemical equations and concepts underlying stoichiometry are well documented; however, there are few classroom-ready instructional solutions to help students build scientifically accurate ideas about these topics central to learning chemistry. An intervention (two inquiry-based activities) was developed, piloted, and evaluated with common misconceptions in mind. The intervention was carried out in five sections of a high school chemistry class at a technical career campus, and pre/posttest data using a published instrument were collected to evaluate the intervention's effectiveness in building accurate stoichiometric concepts. Statistically significant growth with a large effect size occurred from pre to posttest demonstrating that the intervention improved conceptual understanding even though there were variations in the intervention delivery, as well as small differences detected between 11th and 12th grade student performance. The study, an action research project carried out by a teacher enrolled in a long-term professional development program, has implications for the value of rigorous materials design and evaluation framed by the chemistry education research literature. Study replication in other classroom contexts would be useful in further validating the learning outcomes of the activities. For practitioners, the activities studied here are free and available online for classroom use.