Lesson 7: Wrap Up
In this lesson we have seen that the behavior of acids and bases is characterized by reactions whose position of equilibrium is related to the strength of the acids and bases involved. There is, in fact, a predictable balance between the strength of an acid and its conjugate base that is reflected in the position of equilibrium.
We can express the strength of an acid (or a base) mathematically using the equilibrium constant expression and the value of the Ka (or Kb) that applies. This allows us to calculate the pH given the strength and concentration of acid or base present in solution.
The dynamic equilibrium between the concentration of acid and conjugate base is governed also by Le Chatelier's principle, which allows us to predict the response of a system to changes that are imposed from the outside.
We saw that the conjugate base of an acid can be a component of a salt which, when it dissolves in solution, can cause the solution to turn basic. Similarly, salts also may contain the conjugate acid of a base, which can turn solutions of the salt acidic. These hydrolysis reactions can be predicted if you know the relative strengths of the acid and base that would be formed in the reaction.
Finally, we explored the function of buffer solutions, solutions that contain both a weak acid and its (weak) conjugate base. With both species present, the pH of the solution tends to stay constant as the acid neutralizes added base while the base neutralizes added acid. We learned a simple equation that allows us to predict the pH of a buffer solution based on the Ka of the acid and the relative concentrations of the acid and its conjugate base.
Be sure to turn in the lab and the problem set for this lesson and to try the self quiz before you come in to take the quiz. As usual, you'll find the answers to the self quiz below.
This week your lab experiment involves titration, just as it did in the last lesson. This time, however, we'll use titration as a way to explore acid-base equilibria and buffers rather than to determine concentration. The titration itself does not take quite as long as last week's experiment, but you should allow at least one hour to collect the data and an equal amount of time (or even a little bit more) to analyze the data. I recommend that you do your analysis of your experimental data in the lab so that you can ask questions of the lab instructor. You have the option of turning in a group report this week. The graph for the lab report must be done on the computer, not by hand (it is too difficult to be precise enough with a hand-drawn graph). If you need help doing the graph, the lab instructors can assist you.
Self Quiz Answers
1. b
2. b
3. b
4. b
5. a
6. b
7. a
8. d
9. b
10. d