IONIC EQUILIBRIUM CALCULATIONS: A PROBLEM SOLVING APPROACH

Abstract

Students are not able to solve ionic equilibrium problems in a systematic way and using a series of rote-learned formulae is not the best didactic approach. The student has to ask himself or herself questions in order to decide what formula to use: Is this a buffer solution? Are we at the equivalence point? Is the hydrolysis appreciable? Can the dissociation of this weak acid be considered negligible? And so on. These questions bewilder the student (but not the expert chemist) who has to evaluate terms such as "negligible", "perceptible" or "significant". As instructors, we can agree that the difficulty faced by the students in solving such problems is the recognition of the chemical approach required in different situations. Here, a method is presented here that helps the student to develop metacognitive skills, such as planning and trying out of potential problem solutions in qualitative terms before making any calculations. Heuristics for checking the result such as “check the implications of your solution” or "are the units of measurements of the result correct?" are substituted by the more powerful numerical check of the mass balance equation, the electroneutrality condition and the control of unchanged quantity. [Chem. Educ. Res. Pract. Eur.: 2000, 1, 151-160]