This paper investigates the difficulties that Moroccan pupils (18-19) of the second Baccalaureate year encountered in solving chemical equilibrium problems relating to ethanoate ions' reactivity with water and methanoic acid, and to copper-aluminum cells. The pupils were asked to provide answers to questions derived from two problems. The questions were classified into different tasks according to whether their answers required declarative and (or) procedural knowledge. The written responses evaluated and counted as the percentage of successful, failed and unprocessed tasks revealed that pupils completed with success tasks on the reactivity of ethanoate ions with water more easily than those of two other studied cases. The pupils encountered difficulties in appropriating procedural knowledge on equilibriums involved in ethanoate ions' reactivity with methanoic acid and in copper-aluminum cells. The impact of the tasks' organization on knowledge development seems to be not verified as it was shown in the hierarchy prevalence of problems from simple to complex ones.