Common Ion Effect and Buffers in Acid-Base Equilibria

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As in the last assignment, the Review Objectives (think of them as a list of things you should have to do to be successful) are listed first, and the homework questions are below the objectives.  The homework questions for each section of this assignment are listed below a link to the OpenStax textbook that may help you with the homework questions.

This is a 9 point assignment.

OBJECTIVES: After having completed this unit, students should be able to:

1. Define common ion effect and explain how it affects an acid/base or solubility equilibrium.
2. Define, explain, and be able to identify buffer solutions.
3. Explain how buffer solutions are prepared and calculate the amount of the components required to prepare a buffer given pH.
4. Using chemical equations, describe how a buffer solution can resist a change in pH when a small amount of H⁺ (from strong acid) or OH^– (from strong base) is added.
5. Calculate the pH of a buffer solution starting with an initial concentration of the buffer components and K_a or using the Henderson-Hasselbalch equation using the pK_a and the initial buffer component concentrations.
6. Calculate the change in pH of a buffer solution of known composition caused by adding a small amount of strong acid (H⁺) or strong base (OH^–).
7. Define buffering capacity and given a choice of buffers select which one has a higher buffering capacity.

Reading and Homework Exercises

Table of Contents from: OpenStax Chemistry 2e: Section 14.6

14.6 Buffers

Also, read this link from LibreTexts to better understand the common ion effect, paying special attention to the section “Common ion effect with weak acids and bases”

Section 14.6 Practice –

In the previous topic (Acid-Base Chemistry Review), you completed questions covering various acid/base systems, including

• strong acids or bases in water
• weak acids or bases in water
• salts which can undergo hydrolysis (therefore acting as acids or bases), because an ion formed in the dissociation of the salt is a conjugate acid or base.

In this section, you will concentrate on systems that have mixtures containing two of the above (for example, a strong acid/weak acid mixture or a mixture of a weak base and a salt).  Some of these mixtures exhibit what is called the common-ion effect.  This common-ion effect is also the basis for buffer solutions, which are used to stabilize pH (you may remember buffer solutions from CHEM 161 lab).  The first questions test your ability to recognize the common ion effect and buffers and calculate their pH.

Next, there are questions to test your understanding of how buffer solutions resist change in pH, and how much the pH will change when an acid or base is added.

The assignment closes with questions to apply your understanding of buffer capacity and how Ka and Kb values can be used to aid in the selection of comonents user to make a buffer.

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