Concentration of solutions problems are among the most important and at the same time one of the most challenging topics in chemistry. The aim of this study was to determine the sources of the difficulty that students often face when performing calculations related to concentration. The study involved 153 tertiary college students (food science, chemistry and biology majors) who have learned the concepts of mole and concentration of solutions prior to the study. Students' written working steps in answering two concentration questions in the semester summative examination were analyzed. The results showed that 52% of students answered Question 1 correctly whereas only 28% of students solved Question 2 successfully with a clear display of conceptual understanding. Analysis of students' answers led to the identification of common errors, in conceptual understanding of the topics. It was revealed that one contributing factor to the poor success rate in solving problems related to the concentration was the superficial understanding of the mole concept. Mastery of the mole concept is foundational to understanding concentration of solutions. Difficulties with the former may inhibit learning of the latter. However, the results of this study implied that inappropriate application of the formula M[subscript 1]v[subscript 1] = M[subscript 2]v[subscript 2] was the main problem students faced in solving the questions. Specifically, students faced difficulties understanding the use and the meaning of M[subscript 1]v[subscript 1] = M[subscript 2]v[subscript 2]. A lack of conceptual understanding of the mole concept appeared to lead students to rely on the use of memorized formulae and steps to solve the questions. Implications for assessment, research and instruction are also included.