This article presents and discusses the results of a study that investigates university students' comprehension of the concept of molar concentration, following teaching and evaluation of the subject. The specific problems underlying learning of this concept have not yet been focused on in sufficient detail or depth. A "Reasoning with molarity" questionnaire, which explores the logical-mathematical relationships between the variables "n," "V" and "M," was administered to 303 Argentine first-year university students. Results obtained from think-aloud interviews related to this questionnaire, which were performed with 18 students, are also analysed. These results reveal that approximately half the first-year university students have no clear conceptual understanding of molarity. The main difficulty arises in inverse proportionality tasks where the number of moles (extensive) and "M" (intensive) should be related qualitatively to determine which solution occupies the smallest volume. It was also established that much conceptual confusion, such as the lack of differentiation between "n" and "M," lies behind the algorithmic, numerical solution strategy carried out exclusively by many students.