Topic-specific pedagogical content knowledge (TSPCK) enables teachers to transform pedagogically difficult content for specific science topics into forms more easily understood by learners. In science teaching, there is evidence pointing to poor learner performance in key chemistry topics such as stoichiometry. This paper reports on the changes in pre-service teachers' (PSTs) planned TSPCK in stoichiometry. This follows after the practicum that explicitly targeted the development of planned TSPCK for stoichiometry in the methodology class for physical sciences. The study followed a mixed-methods research design and case study as a research strategy. The participants were 10 PSTs in their final year of study for a Bachelor of Education. Quantitative and qualitative data were collected as a set of tests on TSPCK, applied both pre- and post-practicum, using TSPCK tools developed and validated in an earlier study. In both cases, data were analysed using the Rasch method. The distribution of both person and item measures was valid for measuring PSTs' planned TSPCK. In each test, they fell within the conventional fit statistical range of -2 to +2. All of the reliability indices had values [greater than or equal to] 0.6 and hence were considered to be acceptable. The findings in this study indicated a measured improvement in the quality of TSPCK among the PSTs. In addition, PSTs experienced TSPCK components used in planning to teach stoichiometry at different levels of difficulty. This paper recommends further research exploring how such practicum programmes facilitate changes in PSTs' planned TSPCK and what agencies contribute to these changes in planned TSPCK.