Preparation and Evaluation of Buffers (10 Favorites)

LESSON PLAN in Acid & Base Theories, Buffers. Last updated August 17, 2019.


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 pKa 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 pKa 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 pKa of any acid in solution, the ratio between the acid form and base form can be determined (protonation state). If pH < pKa the acid form has a higher concentration than the base form and if pH > pKa the base form has a higher concentration when compared to the acid form.
  • The pH of a buffer is related to both pKa 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.

For the Student

Lesson

Background

In this experiment, we will use two different methods to prepare buffered solutions with the same assigned pH. Buffer 1 will be prepared using acetic acid, HC2H3O2, and sodium acetate, NaC2H3O2.Buffer 2 will be prepared using acetic acid, HC2H3O2, and sodium hydroxide, NaOH. Both buffers will have a target pH of ________. Acetic acid has a Ka = 1.76 x 10-5 and a pKa = 4.75.

Pre-lab Questions

  1. Determine what mass of sodium nitrite, NaNO2, would be required to prepare a buffer, Buffer A, with a pH of 3.13 from 50.0 mL of 1.0 M nitrous acid, NaNO2.The Ka and pKa for nitrous acid is 4.0 x 10-4 and 3.40 respectively.
    1. What is the predominant form of the acid in Buffer A? Explain your answer.
    2. If the pH of Buffer A were 3.40, what would the predominant form of the acid be?
  2. A second buffered solution, Buffer B, is prepared with twice the number of moles of acid and base, but the same ratio of acid to base as Buffer A.
    1. Is the pH of Buffer B greater than, less than, or equal to 3.13?Justify your answer.
    2. Which buffered solution, Buffer A or Buffer B, would be more resistant to pH change when a strong acid or a strong base is added?Justify your answer.
  3. Find step 3 from the Buffer 1 procedure.Perform the calculations required to determine the mass of sodium acetate necessary to produce the buffer of the pH assigned to your lab group.
  4. Find step 3 from the Buffer 2 procedure.Perform the calculations required to determine the mass of sodium acetate necessary to produce the buffer of the pH assigned to your lab group.

Objective

The purpose of this lab is to use two different methods to prepare buffered solutions with the same assigned pH value.

Materials

  • 100 mL 1.0 M HC2H3O2
  • 100 mL 0.50 M NaOH
  • pH meter
  • 50.0 g Sodium Acetate, NaC2H3O2
  • 50 mL Graduated cylinder
  • 00 mL Graduated cylinder
  • 250 mL beaker
  • Balance capable of measuring 0.01 g
  • Weigh boat
  • Stirring rod
  • Disposable pipettes

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
  • When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
  • When diluting acids, always add acid to water.

Procedure for Buffer 1

  1. Using the graduated cylinder, measure 50.0 mL of 1.0 M HC2H3O2. You may want to use a disposable pipette to get an exact volume.
  2. Pour HC2H3O2 into a 250 ml beaker.
  3. Mass out the appropriate number of grams of solid NaC2H3O2 required to reach your assigned pH.
  4. Add the solid NaC2H3O2 to the small beaker and stir with a clean stirring rod until completely dissolved.
  5. Measure the resulting pH using a pH meter and record.

Procedure for Buffer 2

  1. Using the graduated cylinder, measure 50.0 mL of 1.0 M HC2H3O2. You may want to use a disposable pipette to get an exact volume.
  2. Pour HC2H3O2 into a 250 ml beaker.
  3. Measure out the appropriate volume of 0.50 M NaOH required to reach your assigned pH using a clean graduated cylinder.
  4. Add the NaOH to the small beaker and stir with a clean stirring rod.
  5. Measure the resulting pH using a pH meter and record.

Data

Buffer 1 Buffer 2
Assigned pH
Volume of HC2H3O2 (mL)
Mass of NaC2H3O2 (g) N/A
Volume of NaOH (mL) N/A
Actual pH

Calculations

  1. Calculate your percent error using the assigned pH and your actual pH for both Buffer 1 and Buffer 2.

Analysis

  1. If either percent error is greater than 5%, provide a reasonable source of error and explain precisely how the error would affect the actual pH.
  2. A different lab group attempts to make a buffer with an acid and a salt.They choose HCl and NaCl. Explain why their attempt will not produce a solution that is resistant to pH change. Justify your answer using net-ionic equations.
  3. Circle the beaker below that best represents the particulate diagram for Buffer 1. Explain your reasoning referencing the Henderson-Hasselbalch equation.

  1. It is observed that Buffer 2 changes more significantly than Buffer 1 upon the addition of strong acid.Explain this observation.