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Students in grades 11 and 12 are preparing for the annual Advanced Placement (AP) examinations. The tests are the culmination of year-long AP courses where students tackle college-level work. JPA students will be taking AP exams in physics, chemistry, computer science, macro and microeconomics and calculus.

AP Chemistry class has been working hard all year and recently, studied a unit on acid-base equilibria.

AP Chemistry teacher, Ms. Linton, said, “Acid and base equilibrium is one of my favorite units in AP Chemistry. At this point in the year the students have mastered the fundamentals of chemistry and are able to tackle more complex problems independently. This unit is a great opportunity for them to apply what they’ve learned about solution stoichiometry and chemical equilibria to a familiar problem. In my experience, students find this quite frustrating to start with. They have studied ‘acids and bases’ nearly every year since primary school, so there are a lot of preconceptions to overwrite. We’re lucky in JPA to have access to a lot of really great equipment, and this pH investigation challenges the students to work carefully to a high degree of precision – it’s certainly not easy to get right the first time, so multiple trials are required. This is a great lesson in patience, diligence and teamwork - as well as acid base equilibria.”

AP Chemistry students, Sreymach, Soka, Sokhoeun and Sreyrath, reflected on their experiment.

“We recently conducted a lab to investigate the pH of strong and weak acids and bases at different concentrations. In calculating the pH of strong acids or bases, we assume that all of the ions dissociate. As for weak acids and bases, only a small amount of molecules dissociate. So, the actual pH is likely to differ from our calculated value. The experiment challenged our previous understanding of the strength of acids and bases as we try to analyze sources of errors to account for the deviation.

“In our group, we alternated between preparing solutions, calculating expected pH, and taking data readings. We obtained different concentrations of a monoprotic acid (hydrochloric acid), a diprotic acid (sulfuric acid), a strong base (sodium hydroxide), a weak acid (acetic acid), and a weak base (sodium acetate).

“While conducting the tests, we became concerned that our predicted and measured pH values were very different. We checked and re-checked our accuracy when we prepared and diluted the solutions. We also double checked our calculations. We could find no anomalies in our preparation or math, so we checked the equipment and found that our pH probe was faulty. We replaced it and our readings were suddenly in-line with our predictions. These predictions had helped us to identify misleading results due to the fault in our testing equipment and without them, we would have reported wildly inaccurate results. Learning to analyze sources of error between the expected and calculated values is frustrating at times, but it ultimately strengthens our understanding of this important topic.

“A big thank you to Ms. Linton, our AP Chemistry teacher, for giving us the opportunity to conduct our weekly experiments and for preparing us for our upcoming AP exams.”