Chem 1B
Saddleback College
Dr. White
1 Experiment 6 – Properties of a Buffer
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Objectives To review and practice weak acid, weak base equilibrium calculations To understand what a chemical buffer is To investigate different types of acid/base solutions, determining whether they are buffer solutions or not To understand what components are necessary to make a buffer To understand how buffers work to stabilize the pH of a solution Introduction According to Webster’s Dictionary online, a buffer is any (1) person or thing that serves to lessen shock or prevent sharp impact; or (2) any substance in a solution that tends to stabilize the hydrogen ion concentration by neutralizing any added acid or base. This second definition is what we call a chemical buffer. Buffers are used in many ways: to keep the pH level in living organisms constant when drastic changes would imperil life; to control the pH of biological growth media; to maintain pH control in manufacturing processes; to study the effects of drugs; and as acid-­‐base indicators. In this lab you will (1) measure the pH of solutions containing acetic acid, sodium acetate and a combination of acetic acid and sodium acetate; (2) measure the pH of these solutions again after adding a small amount of strong acid or base, and (3) decide whether or not the solutions tested are buffers. Reagents Available 1 M HCl, 1 M HC2H3O2, 1 M NaOH, 1 M NaC2H3O2 The concentrations given above are approximate. The stock solutions will be standardized and the actual concentrations written on the containers. You must record the actual concentrations with correct sig figs in your lab notebook. Procedure As you perform the lab and collect waste solutions pour them into a large beaker. This mixture should then be discarded in the appropriate waste container. DO NOT POUR ANY OF THE SOLUTIONS DOWN THE DRAIN. Use only deionized water for preparing solutions and rinsing the pH electrodes. DO NOT in any circumstance put equipment into reagent bottles or pour any unused reagents back into bottles. Use a SMALL beaker to obtain the quantity of solution needed, and refill the beaker as required. DO NOT WASTE THE CHEMICALS. Using the pH sensor: 1. When not in use, place the pH sensor back into its storage solution. 3. Rinse the pH electrode thoroughly with distilled water and gently pat it dry before placing it in a solution to measure the pH. 4. DO NOT use the pH electrode to stir solutions. 5. As you observe the pH readings allow at least 15 seconds, or until the reading stabilizes before recording your measurements. 6. Read and record the pH values from the computer to two decimal places! (That means two digits after the decimal point.) Part A: Acetic Acid Solution In this part of the lab you will prepare and measure the pH of a 0.10 M acetic acid Chem 1B
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solution. Then you will add HCl and NaOH to it to see how the pH changes. 1. Go to the “Experiment” menu and under “Calibrate” choose the channel with the pH probe (ex: “CH1:pH”). In the window that appears make sure the “Calibration” tab is chosen. Click on “Calibrate Now”. Rinse the pH meter with copious amounts of deionized water. Carefully blot dry. Dip the pH meter in the pH 4 standard solution. Wait until the voltage reading stabilizes. Then, in the field beneath “Enter Value” enter the pH value of the solution and click “Keep”. Repeat the process with the pH 7 standard solution. When finished with this step click on “Done” to close the window. 2. In your lab notebook record the concentration (from the container) of the acetic acid stock solution. Also record the actual concentrations of the approximately 1 M HCl and NaOH stock solutions. 3. Calculate the volume of this acetic acid solution needed to prepare 50.0-­‐mL of 0.10 M acetic acid. Show the calculation in your notebook! 4. Prepare 50.0-­‐mL of 0.10 M acetic acid by diluting the acetic acid stock solution. (You can use your 100-­‐mL graduated cylinder to prepare the solution.) 5. Divide the 0.10 M acetic acid solution into two 25.0-­‐mL portions. Place the two portions into two clean, dry 50-­‐mL beakers. 6. Measure and record the initial pH of one of these solutions. 7. Add 10 drops of the HCl stock solution to one of the portions of the solution and record the pH after mixing. 8. Add 10 drops of the NaOH stock solution to the second portion of the solution and record the pH after mixing. Dr. White
2 9. Pour the two solutions into your waste collection beaker. Part B. Sodium Acetate Solution In this part of the lab you will prepare and measure the pH of a 0.10 M sodium acetate solution. Then you will add HCl and NaOH to it to see how the pH changes. 1. Go to the “Experiment” menu and under “Calibrate” choose the channel with the pH probe (ex: “CH1:pH”). In the window that appears make sure the “Calibration” tab is chosen. Click on “Calibrate Now”. Rinse the pH meter with copious amounts of deionized water. Carefully blot dry. Dip the pH meter in the pH 7 standard solution. Wait until the voltage reading stabilizes. Then, in the field beneath “Enter Value” enter the pH value of the solution and click “Keep”. Repeat the process with the pH 10 standard solution. When finished with this step click on “Done” to close the window. 1. In your lab notebook record the concentration (from the container) of the sodium acetate stock solution. 2. Calculate the volume of this sodium acetate solution needed to prepare 50.0-­‐mL of 0.10 M sodium acetate. Show the calculation in your notebook! 3. Prepare 50.0-­‐mL of 0.10 M sodium acetate by diluting the sodium acetate stock solution. (You can use your 100-­‐mL graduated cylinder to prepare the solution.) 4. Divide the 0.10 M sodium acetate solution into two 25.0-­‐mL portions. Place the two portions into two clean, dry 50-­‐mL beakers. 5. Repeat steps 6-­‐9 from part A with your sodium acetate solution. Chem 1B
Saddleback College
Part C. Acetic Acid and Sodium Acetate Solution In this part of the lab you will prepare and measure the pH of a solution containing equal amounts of acetic acid and sodium acetate. You are to prepare 50.0 mL of a solution that contains both 0.10 M acetic acid and 0.10 M sodium acetate. One way to prepare this solution is by simply mixing appropriate amounts of the stock acetic acid and sodium acetate solutions and then diluting to a total volume of 50.0 mL. However, you are NOT to prepare the solution using this method. There are two other methods that you can use to prepare this solution using the available reagents. Both of these other methods involve using an acid-­base reaction that produces the desired solution (Look at prelab questions #3 and #6 as a hint). Water may also be added to achieve the required final total volume and concentration. As a hint, one of the reactants will be a limiting reactant. You are to determine and use ONE of these other methods. (Think about this before coming to lab!) 1. In your lab notebook write the following: a. The reaction that you will use to produce 50.0 mL of a solution containing both 0.10 M acetic acid and 0.10 M sodium acetate. Choose your reactants from the list of available reagent solutions. b. Identify which reactant needs to be the limiting reactant. c. Record the actual concentrations of the stock solutions you will use. d. Calculate the volume of each stock solution needed. 2. Before preparing the solution, check your calculations with the instructor. 3. Go to the “Experiment” menu and under Dr. White
3 “Calibrate” choose the channel with the pH probe (ex: “CH1:pH”). In the window that appears make sure the “Calibration” tab is chosen. Click on “Calibrate Now”. Rinse the pH meter with copious amounts of deionized water. Carefully blot dry. Dip the pH meter in the pH 4 standard solution. Wait until the voltage reading stabilizes. Then, in the field beneath “Enter Value” enter the pH value of the solution and click “Keep”. Repeat the process with the pH 7 standard solution. When finished with this step click on “Done” to close the window. 4. Prepare the solution. (You can use your 100-­‐mL graduated cylinder to prepare the solution.) 5. Divide the solution into two 25.0-­‐mL portions. Place the two portions into two clean, dry 50-­‐mL beakers. 6. Repeat steps 6-­‐9 from Part A with your acetic acid/sodium acetate solution. Clean-­up 1. Discard any left over reagents and the solution in your waste collection beaker in the appropriately labeled waste container. 2. Rinse the pH electrode, place it back in its storage solution.
Chem 1B
Saddleback College
Dr. White
4 Experiment 6 – Properties of a Buffer Pre-­Lab Assignment 1. Write the chemical equation for the reaction of acetic acid with water. 2. Calculate the pH and percent dissociation of a 0.10 M acetic acid solution. The Ka is 1.8 x 10-­‐5. pH = __________ % Dissociation = ____________ 3. Write the net ionic chemical equation for the reaction that occurs when NaOH(aq) is added to an acetic acid solution. 4. Do you expect the salt sodium acetate to be acidic, basic, or neutral? Write a chemical reaction to explain your choice. Chem 1B
Saddleback College
Dr. White
5 5. Using the reaction you wrote in #4, calculate the pH of a 0.10 M sodium acetate solution. (Hint: You will need to calculate the Kb for the acetate ion. Use the given Ka for acetic acid.) pH = ____________ 6. Write the net ionic chemical equation for the reaction that occurs when HCl(aq) is added to a sodium acetate solution. Chem 1B
Saddleback College
Dr. White
Name: _________________________________________ Lab Day/Time:___________________________ Experiment 6 – Properties of Buffers Data and Results
Preparation of the Solutions: Part A – 0.10 M Acetic Acid solution. Report the amount of each reagent, including any water, used to make the solution. Show how you determined these amounts: Part B – 0.10 M Sodium Acetate Solution. Report the amount of each reagent, including any water, used to make the solution. Show how you determined these amounts: Part C – 0.10 M Acetic Acid/0.10 M Sodium Acetate Solution Write the net ionic chemical equation for the reaction that you used to prepare this solution. Report the amount of each reagent, including any water, used to make the solution: 6 Chem 1B
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Show how you determined these amounts: pH Data: Solution Initial pH Dr. White
pH after adding 10 drops of ________M* HCl pH after adding 10 drops of ________M* NaOH 0.10 M HC2H3O2 0.10 M NaC2H3O2 7 0.10 M HC2H3O2 and 0.10 M NaC2H3O2 *Record the actual concentration of the standardized HCl and NaOH solutions. Questions: 1. A buffer is any substance in a solution that tends to stabilize the hydrogen ion concentration, and thus the pH. Which one of the three tested solutions acted as a buffer? Chem 1B
Saddleback College
Dr. White
8 2. This solution acts as a buffer due to reactions that occur within the solution when an acid or a base is added. a. Write the net ionic chemical equation for the reaction that occurs within this buffer solution when HCl(aq) is added. b. Write the net ionic chemical equation for the reaction that occurs within this buffer solution when NaOH(aq) is added. 3. Consider a solution containing equal concentrations of HCl and NaCl. Could this solution be used as a buffer? Explain why or why not. (Hint: think about what reactions would need to occur to stabilize the pH as an acid or base is added to the solution.) 4. What two types of components must be generally present in a solution to enable the solution to act as a buffer? 5. Identify which of the following will result in a buffer solution when equal volumes of the two solutions are mixed. (Circle all that apply.) (a) 0.10 M NaNO2 and 0.10 M HNO2 (b) 0.10 M HCl and 0.10 M NH3 (c) 0.10 M KNO3 and 0.10 M HNO3 (d) 0.20 M HCl and 0.10 M NH3 (e) 0.10 M HCl and 0.20 M NH3 Chem 1B
Saddleback College
Dr. White
9 6. In the solution containing both 0.10 M acetic acid and 0.10 M sodium acetate, the acetic acid undergoes dissociation. The chemical equation for this dissociation reaction is the same as for a solution containing acetic acid alone. The difference is that the initial concentration of acetate ion for the solution containing acetic acid alone is zero, whereas the initial concentration of acetate ion is 0.10 M in your solution containing both acetic acid and sodium acetate. Calculate the percent ionization and the expected initial pH for the solution that contained both 0.10 M acetic acid and 0.10 M sodium acetate. (Hint: You will need to use Ka for acetic acid given in the prelab.) pH = __________ % Dissociation = _______________ 7. Compare the calculated percent ionization of acetic acid in the solution containing both 0.10 M acetic acid and 0.10 M sodium acetate to the calculated percent ionization for the solution containing only 0.10 M acetic acid (as determined in #2 of the prelab). In which solution is the percent ionization the lowest? Explain why this occurs.