Undergraduate mathematic students (UMS) who easier to understand logarithmic knowledge are required to be analysed their understanding on conceptual knowledge. General chemistry lecture contains conceptual knowledge as microscopic representation not been observed by them. UMS’ scientific reasoning can affect their ability to do abstraction of microscopic phenomenon. Based on Piaget’s theory of cognitive development, UMS are in operational formal category who  have ability to do concept abstraction. However the result of scientific reasoning skill by Lawson Classroom Test of Scientific Reasoning (LCTSR) instrument was found that 2 students of 28 UMS whose have scientific reasoning as formal category. The aim of this research was analysing UMS’ achievement on understanding conceptual knowledge which contains microscopic representation based on their scientific reasoning (concrete, transition, or formal category).  The research funding that mean score of  UMS’ scientific reasoning as formal category (middle test 79,1; final test 90) were higher than UMS’ scientific reasoning as transition category (middle test 73,5; final test 81,2) and UMS’ scientific reasoning as concrete category (middle test 62,8; final test 75,9). The other fundings described that majority of  UMS had well done to solve logarithmic question, but they were difficult to solve  conceptual question which contains microscopic representation. Sample question about how to determine limiting reactant based on molecule representation on before and after reaction. On detail that 25% of  UMS’ scientific reasoning as concrete category (UMSC); 14,3% UMS’ scientific reasoning as transition category (UMST); 3,6% UMS’ scientific reasoning as formal category (UMSF), They written number of molecules on chemical equation which is not as net reaction. On other response that 10,7% of  UMSC answered there was no limiting reactant. The characterictic of  UMSC is understanding based on object which they observed. Molecules was observed concretely through figure on before and after reaction, meanwhile they difficult to transform it as microscopic representation (number of molecules as reactant and product) which was written on net reaction as symbolic representation. They did not consider stoichiometric chemical equation by using representation of  X as a variable in  mathematical calculations.  Another response that 14,3% of  UMST determined limiting reactant by using logical mathematic equation of mole=mass/molecule relative mass without considered coefficient in net reaction. They justified limiting reactant which has higher molecule relative mass, its answer could not accepted scientifically. Their scientific reasoning ability can affect how they understood microscopic representation in general chemistry lecture. Majority of UMS who have not formal category of scientific reasoning ability was needed scaffolding to vusialize microscopic phenomenon.

Keywords: Understanding Microscopic Representation, Scientific Reasoning Ability, and General Chemistry