Learners underperform in stoichiometry as they lack conceptual reasoning of the underlying concepts and the ability to solve stoichiometric problems. Therefore, it was necessary to determine if there is a statistical correlation between problem-solving skills and conceptual reasoning in stoichiometry and if so, whether one can significantly predict the other. The theoretical framework is the cognitive load theory (CLT). This theory expects teachers to know where to focus their teaching and how to assess their learners' work to avoid unnecessary overloading of the working memory, which might affect their performance. The explanatory sequential mixed-method research design was employed with 410 grade 11 Physical Science learners in their intact classes. The participants wrote the learner achievement test (LAT) and responded to a semi-structured interview. The learners' test scores were then used to run a statistical test. The Pearson correlation and regression showed that the justifications given by learners for choosing correct or incorrect multiple-choice options were not due to chance, and the results of the learner interviews supported the learners' performance in the test. Moreover, the findings indicated that there was a positive correlation between problem-solving skills and conceptual reasoning where statistically, conceptual reasoning predicted learners' problem-solving skills using regression analysis.