Analyzes the role of students' prior knowledge in their emerging understanding of the mole. The research question this study seeks to answer is what knowledge, if any, do student have regarding the mole and what prior knowledge do they access when presented problems regarding the nature of the mole. Data collection focuses on student knowledge regarding the mole in three areas; 1) math skills and number sense, 2) the mole and 3) amount of substance. It was found that many students relied on observable properties of matter at a macroscopic scale in their reasoning about amount of substance. Reliance on observable properties of materials, like whether the substance is a solid versus liquid, or the surface features of a problem like the number quantities in chemical formulas, plays a large role in student reasoning. The implication of this research is that students may not be able to develop a conceptual understanding of the mole unless they shift their understanding from a macroscopic view of matter to a particulate model of matter. (Author)