This paper describes the implementation of problem-based learning in chemical education with regard to the impact that protolytic reactions have on equilibria. The problem-based task presented here is focused on extremely weak acids and calcuation of the pH value of their aqueous solutions. The task is based on comparisons of K[subscript a] ranges over which calculations using the universal cubic equation, quadratic equation with a nonlinear term, and the simplest equation for pH calculation (pH = -log[H[superscript +]]) are each valid. Our students observed that an extremely weak acid can be defined as an acid with a pK[subscript a] greater than 8.13, with a relative error of 0.005 pH units, or 7.89, with a relative error of 0.01 pH units. Under these conditions, the quadratic equation without linear term serves as a universal formula. Students then solved a criterial equation using a quadratic equation without a linear term and the simplest formula to estimate the critical concentration, which is dependent on the pK[subscript a] value. Below this concentration, the simplest formula cannot be used. During practical applications, students observed that the critical concentration for hydrogen peroxide is very high (0.193 mol L[superscript -1]) and that the pH of its aqueous solution should be calculated according to a quadratic equation without a linear term in most of the practical examples. When traditional educational methods were used, students were unable to solve this problem properly, so we searched for a new way to solve this task. Taking into account the principles of problem-based learning, we designed a didactic cycle (as represented by a flowchart) for solving the above-mentioned problem. In this cycle, learning was aimed at the students, and the teacher only served as a facilitator. In this situation, the students' own work, research, and discovery were highlighted, together with the development of their chemical, mathematical, and IT skills.