This paper documents the pedagogical content knowledge (PCK) of four university professors in General Chemistry for the topic "amount of substance"; a fundamental quantity of the International System of Units (SI). The research method involved the development of a Content Representation and the application of Mortimer's Conceptual Profile Model to evaluate the way in which participants structured their knowledge of the topic. Five conceptual profile zones were defined: perceptive/intuitive, empiricist, formalist, rationalist, and formal rationalist. These zones were then used as criteria to classify participants' PCK as articulated in the framework of their Content Representations. As a consequence, four different conceptual profile graphs were constructed by plotting the percentage of times that each conceptual profile zone appeared. These conceptual profile graphs revealed the ways in which each professor conceptualised his or her teaching of "amount of substance." (Contains 2 figures, 2 notes and 3 tables.)