In this paper, we present and analyze a virtual-lab based activity oriented as an application of the stoichiometry concepts that appear in the first year of the Spanish Bachillerato (16-17 year old). The activity was brought to class and was used by more than hundred Catalan high school students. The task was worked in pairs and each action they did in the virtual lab was anonymously recorded to a log file. The analyses of these log file allow us to affirm that this type of activities provide a significant support for a focused and active learning experience.