This paper reports a comparison of a group of higher-and lower-achieving undergraduate chemistry students, 17 in total, as separated on their ability in stoichiometry. This exploratory study of 17 students investigated parallels and differences in the students' general and domain-specific cognitive abilities. Performance, strategies, and mistakes in the students' problem solving in the field of stoichiometry were also investigated in detail. The study revealed that the major difference between higher-and lower-achieving students lies in their cognitive skills, especially in domain-specific (mole concept) skills and the ability to deal with complexity. Results suggest that a thoroughly differentiated set of tasks be applied in the undergraduate chemistry classroom: tasks with limited complexity and structured help or scaffolding are needed for lower-achieving students, whereas complex and abstract tasks are needed to challenge the higher-achieving students.