2. Calculate the pH of a solution that is 0.147 M in pyridine and 0.189 M in pyridinium chloride.
Before beginning this problem, spend a few minutes talking about nomenclature of acids, bases, cations and anions, including the suffixes that are used to denote negatively (“ate”, “ide”, “ite”) and positively (“ium”) charged species.
Write the correct chemical equation that represents this solution at equilibrium.
Allow the students about ten minutes to get started on this problem. Many students may be tempted to have both pyridine and pyridinium appear as reactants in the same chemical equation. Talk about the possibility of having two expressions representing this system. Groups are then usually able to write both the Ka and Kb expressions that could be used to describe the system. Sometime during this discussion the students need to think about the following two questions:
What happens to the pyridinium chloride when dissolved in water? What is the role of Cl- in the solution?
Talk for about five minutes about spectator ions and their role in the system.
Instruct the students to pick one equation (either Ka or Kb) and use it to find the pH of the solution using the system established in problem 1.
It may take them a while to arrive at a pH that they are satisfied with. Some may need a reminder that there are initial amounts of both pyridine and pyridinium. If some groups finish before others instruct them to try finding the pH using the other equation.
Are they two pH values the same? Is this a surprise?
How does the magnitude of Ka compare to the magnitude of Kb? What does this tell us about which equation we should use?
Discuss how both reactions cannot be going towards product at the same time, so there must be one expression that more accurately represents this system.
What is the relationship between the two species present?
Talk for about fifteen minutes about conjugate pairs and buffers. Talk about what sorts of factors influence the buffer capacity of a system. Introduce the Henderson-Hasselbalch expression. Discuss how by examining the form of the Henderson-Hasselbalch equation, we can explain why diluting a buffer does not lead to a change in pH (unless the solution becomes excessively dilute – which then is no longer really a buffer).
Is the pH found using the Henderson-Hasselbalch expression the same as the one found before?