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Titration Results Trial 1 Trial 2 Equivalence point pH 2.35 8.28 pH of half-titrated solution 4.86 4.11 1. Calculate the pKa and Ka using the results of your testing. 2. Find the accepted values for the pKa and Ka of acetic acid. How well do the accepted values compare with your calculated values? Explain. 3. Explain why the pH at the half-titration point is equal to the pKa in your experiment using the Henderson-Hasselbalch equation. 4. Explain how this test could be done using only an indicator solution and no electronic means of measuring pH. DATA ANALYSIS Trial 3 8-20 4.43

Titration Results Trial 1 Trial 2 Equivalence point pH 2.35 8.28 pH of half-titrated solution 4.86 4.11 1. Calculate the pKa and Ka using the results of your testing. 2. Find the accepted values for the pKa and Ka of acetic acid. How well do the accepted values compare with your calculated values? Explain. 3. Explain why the pH at the half-titration point is equal to the pKa in your experiment using the Henderson-Hasselbalch equation. 4. Explain how this test could be done using only an indicator solution and no electronic means of measuring pH. DATA ANALYSIS Trial 3 8-20 4.43

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ISBN:9781305957404
Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
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Determining Ka by the Half-Titration of a Weak Acid A common analysis of a weak acid or a weak base is to conduct a titration with a base or acid of known molar concentration to help determine the equilibrium constant, Ka, for the weak acid or weak base. If this titration is conducted very carefully and very precisely, the results can lead to a valid approximation of an equilibrium constant. In this experiment, however, you will use a different technique to determine the Ka for a weak acid, acetic acid. Your primary goal in this experiment is to calculate the Ka of acetic acid. The data that you will use to complete your calculations will come from the reaction of acetic acid with a solution of NaOH. Recall from your work with weak acid-strong base titrations that the point at which a reaction is half-titrated can be used to determine the pKa of the weak acid. In this experiment, the half-titration point will exist when you have added half as many moles of HC₂H3O2 as moles of NaOH. Thus, OH- will have reacted with half of the HC₂H302, leaving the solution with equal moles of HC2H3O2 and C2H3O2-. At this point, according to the Henderson- Hasselbalch equation, pH = pk + log. [C₂H₂O₂] [HC,H,O,] (7 if there are equal moles of HC₂H3O2 and C2H3O₂ at the half-titration point, then pKa is equal to the pH value of the solution. In this experiment, you may find it surprising that you do not need to keep close track of the volume of NaOH titrant added, as you would in most titrations. It is also unusual to conduct a titration without plotting or analyzing a conventional titration curve. This is the nature of a half-titration; it is only important to know when equal amounts of OH and HC₂H3O2 have been added. OBJECTIVES In this experiment, you will Conduct a reaction between solutions of a weak acid and sodium hydroxide. Determine the half-titration point of an acid-base reaction. Calculate the pKa and the Ka for the weak acid. Figure 1 1.00 M sodium hydroxide, NaOH, solution 1.00 M acetic acid, HC₂H3O2, solution phenolphthalein indicator solution distilled water magnetic stirrer and stirring bar plastic Beral pipets utility clamp MATERIALS LabQuest LabQuest App Vernier pH Sensor 50 mL buret buret clamp 250 mL beaker two ring stands 11
Transcribed Image Text:Determining Ka by the Half-Titration of a Weak Acid A common analysis of a weak acid or a weak base is to conduct a titration with a base or acid of known molar concentration to help determine the equilibrium constant, Ka, for the weak acid or weak base. If this titration is conducted very carefully and very precisely, the results can lead to a valid approximation of an equilibrium constant. In this experiment, however, you will use a different technique to determine the Ka for a weak acid, acetic acid. Your primary goal in this experiment is to calculate the Ka of acetic acid. The data that you will use to complete your calculations will come from the reaction of acetic acid with a solution of NaOH. Recall from your work with weak acid-strong base titrations that the point at which a reaction is half-titrated can be used to determine the pKa of the weak acid. In this experiment, the half-titration point will exist when you have added half as many moles of HC₂H3O2 as moles of NaOH. Thus, OH- will have reacted with half of the HC₂H302, leaving the solution with equal moles of HC2H3O2 and C2H3O2-. At this point, according to the Henderson- Hasselbalch equation, pH = pk + log. [C₂H₂O₂] [HC,H,O,] (7 if there are equal moles of HC₂H3O2 and C2H3O₂ at the half-titration point, then pKa is equal to the pH value of the solution. In this experiment, you may find it surprising that you do not need to keep close track of the volume of NaOH titrant added, as you would in most titrations. It is also unusual to conduct a titration without plotting or analyzing a conventional titration curve. This is the nature of a half-titration; it is only important to know when equal amounts of OH and HC₂H3O2 have been added. OBJECTIVES In this experiment, you will Conduct a reaction between solutions of a weak acid and sodium hydroxide. Determine the half-titration point of an acid-base reaction. Calculate the pKa and the Ka for the weak acid. Figure 1 1.00 M sodium hydroxide, NaOH, solution 1.00 M acetic acid, HC₂H3O2, solution phenolphthalein indicator solution distilled water magnetic stirrer and stirring bar plastic Beral pipets utility clamp MATERIALS LabQuest LabQuest App Vernier pH Sensor 50 mL buret buret clamp 250 mL beaker two ring stands 11
Titration Results Trial 1 Trial 2 Trial 3 Equivalence point pH 8.35 3.28 8.90 pH of half-titrated solution 4.86 4.17 4.73 MAYONETAZZ 1. Calculate the pKa and Ka using the results of your testing. 2. Find the accepted values for the pKa and Ka of acetic acid. How well do the accepted values compare with your calculated values? Explain. 3. Explain why the pH at the half-titration point is equal to the pKa in your experiment using the Henderson-Hasselbalch equation. 4. Explain how this test could be done using only an indicator solution and no electronic means of measuring pH. DATA ANALYSIS
Transcribed Image Text:Titration Results Trial 1 Trial 2 Trial 3 Equivalence point pH 8.35 3.28 8.90 pH of half-titrated solution 4.86 4.17 4.73 MAYONETAZZ 1. Calculate the pKa and Ka using the results of your testing. 2. Find the accepted values for the pKa and Ka of acetic acid. How well do the accepted values compare with your calculated values? Explain. 3. Explain why the pH at the half-titration point is equal to the pKa in your experiment using the Henderson-Hasselbalch equation. 4. Explain how this test could be done using only an indicator solution and no electronic means of measuring pH. DATA ANALYSIS
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