Summary

In this lesson students will use multiple methods to calculate and prepare buffered solutions with a desired pH. Upon preparation of the solutions, the students will explore differing aspects of buffers including buffering capacity and predominant form.

Grade Level

High School (AP Chemistry)

AP Chemistry Curriculum Framework

This activity supports the following unit, topics and learning objectives:

• Unit 8: Acids and Bases
  • Topic 8.4: Acid-Base Reactions and Buffers
    • SAP-9.D: Explain the relationship among the concentrations of major species in a mixture of weak and strong acids and bases.
  • Topic-8.8: Properties of Buffers
    • SAP-10.B: Explain the relationship between the ability of a buffer to stabilize pH and the reactions that occur when an acid or a base is added to a buffered solution.
  • Topic-8.9: Henderson-Hasselbalch Equation
    • SAP-10.C: Identify the pH of a buffer solution based on the identity and concentrations of the conjugate acid-base pair used to create the buffer.
  • Topic 8.10: Buffer Capacity
    • SAP-10.D: Explain the relationship between the buffer capacity of a solution and the relative concentrations of the conjugate acid and conjugate base components of the solution.

Objectives

By the end of this lesson, students should be able to

• Prepare a buffered solution with a desired pH from a weak acid and its salt.
• Prepare a buffered solution with a desired pH by partially neutralizing a weak acid with a strong base.
• Compare the buffering capacity between two buffered solutions.
• Evaluate the predominant form of an acid in a solution of a specific pH.

Chemistry Topics

This lesson supports students’ understanding of

• Acids & Bases
• Acid – Base Theory
• Buffers
• pH
• Buffering capacity

Time

Teacher Preparation: 30 minutes

Lesson: 90-120 minutes

Materials per group

• 100 mL 1.0 M HC₂H₃O₂
• 100 mL 0.50 M NaOH
• pH meter or pH paper
• Approximately 5 g Sodium Acetate, NaC₂H₃O₂
• 50 ml Graduated cylinders
• 50 ml Graduated cylinders (50 mL and 100 mL)
• 250 mL beaker
• 1- 5.00 mL pipette
• Balance capable of measuring to 0.01 g
• Weighing boats

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
• When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
• When diluting acids, always add acid to water.

Teacher Notes

• Lesson Outline:
• Day 1 (45-60 minutes): Use the short PowerPoint provided to cover all of the concepts that the students will be addressing during the lab. There is also a formative quiz that can be used with students to determine the level of readiness to proceed with the lab. Also the Pre-lab section of the lab should be completed and discussed by the lab groups. The Pre-lab section does not appear to be very time consuming at first glance, however, these calculations are some of the most troubling and challenging that the students will encounter. Ensuring that they can successfully complete them is worth dedicating class time, rather than assigning them for homework.
• Day 2 (45-60 minutes): The lab activity should be completed, post lab questions should be answered and a consensus should be reached within each group.
• This lab is designed to help students understand one of the more challenging ideas in AP Chemistry: buffers and buffering capacity. It was designed with free response question 3 from the 2017 administration of the AP Chemistry Exam in mind. Some parts of this FRQ would be an excellent way to determine if students understood the concepts from this lab.
• The teacher will need to either purchase standardized solutions of 1.0 M HC₂H₃O₂ and 0.50 M NaOH solutions or prepare and standardize the solutions.
• In this lab students will use two different methods to prepare buffered solutions with the same pH. Buffer 1 is prepared using a weak acid, acetic acid, and its salt, sodium acetate. Buffer 2 is prepared by partially neutralizing a weak acid, acetic acid, with a strong base, sodium hydroxide.

• Student lab groups of 3 can be assigned varying target pH values to promote each lab group to complete their own calculations. This can be done by varying the assigned pH values as follows: Group 1 pH = 4.50, Group 2 pH = 4.55, Group 3 pH = 4.60, etc.
• The pK_a of acetic acid is 4.75, so you may or may not want to assign a group the value of 4.75.
• Download the Excel spreadsheet for this resource to calculate the mass of sodium acetate needed for Buffer 1 and the volume of sodium hydroxide needed for Buffer 2.
• This lab should be completed once students are comfortable with all of the AP essential knowledge regarding buffer calculations, and the concept of buffering capacity which are outlined below.

• Buffer Background Information:
• Essential Knowledge 6.C.2: The pH is an important characteristic of aqueous solutions that can be controlled with buffers.Comparing pH to pK_a allows one to determine the protonation state of a molecule with a labile proton.
• The first point goal of teaching buffers is recognizing what a buffer is composed of. In order for a solution to be classified as a buffer it must contain both members of a conjugate acid-base pair.This allows any added base to react with conjugate acid and any added acid to react with conjugate base.
• By comparing pH to pK_a of any acid in solution, the ratio between the acid form and base form can be determined (protonation state). If pH < pK_a the acid form has a higher concentration than the base form and if pH > pK_a the base form has a higher concentration when compared to the acid form.
• The pH of a buffer is related to both pK_a as well as the ratio of acid and base forms (evidenced by the Henderson-Hasselbalch equation). The buffer capacity is related to absolute concentration of the acid and base forms.Therefore, it is possible for two buffers of equal pH to respond differently to the addition of a strong acid, or strong base, therefore have a differing buffer capacity.
• Past Free-Response Questions Relating to These Concepts:

• An old FRQ to get the students used to the calculations involved in the process would be the 2002 Form B #1. It is pretty straight forward but does get them used to the equations and processes needed.
• Good predominant form question 2013 secure practice exam #7. This question does get convoluted with interparticle force ideas, but the predominant form ideas are well done.Since it is on the secure exam a link cannot be provided.
• Another great predominant form question can be found on the 2016 released exam question number 4. According to the Student Performance Q & A, most common predominant form mistake made by students was to assume at any pH greater than 7 the acid would be in its basic form.
• One of the better buffer capacity questions can be found on the 2007 AP Chemistry Exam Form B Question#5 ci-iii.
• The 2011 FRQ #1 was a really good question for this concept, but part c is now in an exclusion statement.This does really do a good job of addressing what a buffer is.