Problem-solving is one of the main goals in science teaching and is something many students find difficult. This research reports on the development, implementation and evaluation of a problem-solving heuristic. This heuristic intends to help students to understand the steps involved in problem solving (metacognitive tool), and to provide them with an organized approach to tackling problems in a systematic way. This approach guides students by means of logical reasoning to make a qualitative representation of the solution of a problem before undertaking calculations, using a "backwards strategy," which thus comprises a cognitive tool. The findings of the study suggest that students found the heuristic useful in setting up and solving quantitative chemical problems, and helped them to understand the phases of the problem solving process. Possible applications of the heuristic in the classroom include its use in formative assessment, to identify and to overcome student alternative conceptions, problem-solving in a cooperative environment, and to reduce the gender gap in science. (Contains 6 figures.)