Week Two Buffer Solutions Experiment Summary

WEEK TWO PROCEDURE PART I — PREPARING BUFFER SOLUTIONS

• Students work in groups of three.

• Groups work with conjugate bases from Week One:

  • Sodium acetate ($CH3CO2Na$)

  • Sodium formate ($HCO_2Na$)

  • Sodium propanoate ($CH3CH2CO_2Na$)

• Prepare three buffer solutions by mixing assigned conjugate base with HCl as follows:

  • Table B:

  • Buffer 1: 25.00 mL $0.50 M$ base, 25.00 mL $0.25 M$ HCl, 0.00 mL H2O

  • Buffer 2: 25.00 mL $0.50 M$ base, 17.50 mL $0.25 M$ HCl, 7.50 mL H2O

  • Buffer 3: 25.00 mL $0.50 M$ base, 12.50 mL $0.25 M$ HCl, 12.50 mL H2O

• Steps for preparation:

  1. Clean and label three 100-mL beakers: Buffer 1, Buffer 2, Buffer 3.

  2. Add 25.00 mL of the 0.50 M conjugate base to each beaker.

  3. Use a burette to add the appropriate volume of $0.25 M$ HCl to each beaker.

  4. Use a burette to add water.

PART II — MONITORING THE pH OF THE BUFFERS

• Perform titrations on each buffer prepared above.

• Each group member takes turns measuring/recording $NaOH$ volumes and pH values.

• For each buffer, repeat the following:

  1. Create a data table (record mL of $NaOH$ and pH).

  2. Measure initial pH with a pH meter (0.0 mL $NaOH$).

  3. Add 1-mL aliquots of $0.10 M$ $NaOH$ and measure pH after each addition until significant change in pH.

IN-LAB DISCUSSION

• Effects of adding strong base on buffer pH.

• Effect of decreasing HCl in buffer preparation on pH.

• Identify the buffer offering the best pH protection.

REPORTING PROCEDURE – WEEK TWO

1. Prepare report on separate sheets using full sentences.

2. Summarize data in tables for titrations, showing volume of strong base and pH.

3. Write balanced equation for net ionic reaction used in buffer preparation.

4. Calculate initial concentrations of the weak base and conjugate acid.

5. Predict pH using literature values for $Ka$ or $Kb$; compare with experimental data.

6. Create graphs showing pH vs. strong base volume added.

7. Calculate average moles of strong base needed to change pH by one unit for each buffer.

8. Explain the relationship between averaging moles of strong base and [base]/[acid] ratio.

REPORTING PROCEDURE – COMPARING WEEKS 1 AND 2

• Compare results and similarities/differences in buffers created in Weeks 1 and 2.

• Discuss titrations from both weeks.

• Reflect on key learnings about buffers' resistance to pH changes from both weeks.

• Consider necessary information to prepare an effective buffer based on lab experiences.

WEEK TWO PROCEDURE PART I — PREPARING BUFFER SOLUTIONS

Students work in groups of three and utilize conjugate bases from Week One, specifically sodium acetate ($CH3CO2Na$), sodium formate ($HCO2Na$), and sodium propanoate ($CH3CH2CO2Na$). Each group is tasked with preparing three buffer solutions by mixing their assigned conjugate base with HCl according to the specified volumes detailed in Table B. For Buffer 1, the mixture consists of 25.00 mL of $0.50 M$ conjugate base, 25.00 mL of $0.25 M$ HCl, and no additional water. Buffer 2 includes 25.00 mL of $0.50 M$ base, 17.50 mL of $0.25 M$ HCl, and 7.50 mL of water. Lastly, Buffer 3 is prepared using 25.00 mL of $0.50 M$ base, 12.50 mL of $0.25 M$ HCl, and 12.50 mL of water.

To prepare the buffers, groups must first clean and label three 100-mL beakers designated for each buffer. They will then add 25.00 mL of the selected $0.50 M$ conjugate base to each beaker, followed by the addition of the appropriate volume of $0.25 M$ HCl using a burette. Finally, they will use a burette to add water as needed.

PART II — MONITORING THE pH OF THE BUFFERS

In the second part of the experiment, groups perform titrations on each prepared buffer. Each member takes turns measuring and recording the volumes of $NaOH$ added and the corresponding pH values. The process involves creating a data table to log the mL of $NaOH$ and pH for each buffer. Initially, the pH is measured using a pH meter without any added $NaOH$. Subsequently, 1-mL aliquots of $0.10 M$ $NaOH$ are added, with the pH being measured after each addition until there is a significant change in pH.

IN-LAB DISCUSSION

During the in-lab discussion, students will explore the effects of adding a strong base on buffer pH, the impact of decreasing HCl during buffer preparation on pH, and identify which buffer provides the best pH protection.

REPORTING PROCEDURE – WEEK TWO

For the reporting procedure, students must prepare reports on separate sheets utilizing full sentences and summarize their titration data in tables that show the volume of strong base added and corresponding pH levels. Additionally, they should write the balanced equation for the net ionic reaction used in the buffer preparation and calculate the initial concentrations of the weak base and conjugate acid. Students will predict pH using literature values for $Ka$ or $Kb$ and compare their findings with experimental data. Graphs illustrating pH against the volume of strong base added must be created, along with calculations for the average moles of strong base required to alter the pH by one unit for each buffer. Lastly, they should explain the relationship between the average moles of strong base and the [base]/[acid] ratio.

REPORTING PROCEDURE – COMPARING WEEKS 1 AND 2

In this section, students will compare the results and examine the similarities and differences in the buffers created during Weeks 1 and 2. They should discuss titration results from both weeks and reflect on key learnings regarding buffers' resistance to pH changes. Finally, students will consider the essential information needed to prepare an effective buffer based on their laboratory experiences.

In week 2, we will prepare three buffer solutions using conjugate bases from week one along with varying amounts of HCl and water. This is so we can establish different base/acid ratios. We will then titrate each buffer with NaOH, recording pH changes to analyze buffering capacity. Our goal is to understand how buffer composition affects resistance to pH changes.