The purpose of the study was to investigate grade 11 learners’ problem solving skills and conceptual reasoning on concepts in stoichiometry. Two theoretical frameworks were used in this study namely, cognitive load theory (CLT) and typology of curriculum representation (levels of curriculum alignment). The explanatory sequential mixed method research design were applied where 410 physical sciences learners in their intact classes and eleven of their teachers participated in the study. The participants completed purposefully designed research instruments consisting of an achievement test (LAT), a teacher and a learner questionnaire (TSQ and LCQ), and a teacher lesson plan on stoichiometry-related concepts (TLP). Other instruments used include a semi-structured interview schedule (LIS), classroom observation schedule (COS) and learner work books (LWB). The study was underpinned by five research questions. Pearson correlations showed that the justifications given by learners for choosing right or wrong objective options were not due to chance and suggested a learner choosing the right objective option has the right conceptual reasoning. Findings indicate that there is a positive correlation between problem-solving skills and conceptual reasoning where conceptual reasoning statistically predicted learners’ problem-solving skills using Regression. Problem solving is an important cognitive activity in everyday and professional contexts. Therefore, it requires teachers to know where to focus their teaching and how to assess learners’ work to avoid unnecessary overloading of the working memory of learners which might affect their performance. Conceptual reasoning and problem-solving errors were made during the problem solving, for example, learners could not apply mole ratio, they were unable to do change of subject and they interchanged the meanings of chemical terms such as mole, molecule, atoms and mass. In terms of possible reasons for the errors, and how it linked to the way learners were taught, the curriculum levels were considered. Findings indicate that there is a mismatch between what is expected in stoichiometry from the CAPS curriculum termed intended curriculum and the implemented curriculum which includes teacher lesson plans, classroom observations and learner workbooks. The classroom observations schedule was designed in line with the teacher lesson plan. There were topics planned in the lessons that were not found in the learner workbooks. The analysis of the content of learners’ workbooks for topics treated under stoichiometry, revealed that 75.7% of concepts were being taught while 24.3% were not. The analysis showed that learners experienced difficulties with calculating the concept of limiting reactant. There is a mismatch between implemented and attained curriculum as well. In the TSQ, teachers indicated that the errors were due to misconceptions, misunderstanding, carelessness and misinterpretation. These were found during marking of the learner achievement test. In conclusion, this study did not find a direct link between the way the learners were taught and the errors they committed during problem solving based on the implemented (TLP, LWB and COS) and attained curriculum (LAT, LCQ, LIS and DoBE report) / Science and Technology Education / Ph. D. (Mathematics, Science and Technology Education)