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
Chapter15: Acid-base Equilibria
Section: Chapter Questions
Problem 7RQ: Sketch the titration curve for a weak acid titrated by a strong base. When performing calculations...
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Question
![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](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2Fbb8a976c-eb7b-4ed9-b029-cfa72f795dab%2F9c39ada1-72bb-4272-babf-8d5c338f75a8%2F2jr8fn_processed.jpeg&w=3840&q=75)
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
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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