IN-DEPTH ONLINE LAB INVESTIGATION:
ACID-BASE TITRATIONS
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Can you……
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give the Arrhenius and Brønsted–Lowry definitions for acids and bases?
remember acid-base nomenclature?
describe reactions of acids and bases in aqueous solutions?
calculate and explain the concept of pH?
describe the dissociation processes that occur in aqueous solutions for
acids and bases (strong and weak)?
□ write a complete balanced equation, ionic equation, and net ionic equation
for a chemical reaction that occurs in aqueous solution?
□ use a scientific/graphing calculator to perform calculations involving
logarithms?
□ interpret graphical representations of data?
What is a titration?
What purpose does it serve?
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BACKGROUND READING…..
Titrations
Suppose that you were to dispense 0.500 M sodium hydroxide (NaOH) from a buret into an
Erlenmeyer flask containing 50.0 mL of hydrochloric acid (HCl). Not only would a neutralization
reaction occur, but careful measurements would enable you to determine the concentration of the HCl
solution. Such an analysis, termed a titration, is used to determine the unknown concentration of an
acid or base by reacting it with a known concentration of acid or base.
During the titration process, as the acid and base neutralize one another, a point is reached (the
equivalence point) where the moles of H+ and OH- ions in the solution are equal. After the
equivalence point is reached, continued addition of acid or base creates an excess of H + or OH- ions
in the solution.
Indicators
How would you be able to recognize the equivalence point? An indicator is a substance that changes
color due to the presence of excess acid or base. If an appropriate indicator is added to the
Erlenmeyer flask before the titration begins, it will change color after the equivalence point, in
response to the excess H+ or OH- ions. The indicator signals the endpoint of the titration.
Choice of Indicators
It is important to note that each indicator has a specific pH range over which it changes color. Thus,
each titration must use an indicator that changes color near the equivalence point pH (which can be
estimated for each type of titration). For example, bromthymol blue could be used as an indicator for
a titration whose equivalence point pH is 6.3, whereas phenolphthalein might act as the indicator for a
titration whose equivalence point pH is 9.0.
You will explore three types of titrations, using the ChemScope—titration of a strong acid with a
strong base, titration of a weak acid with a strong base, and titration of a weak base with a strong
acid.
Data & Observations
Part 1: Titration of Strong Acid with Strong Base
Use the ChemScope to examine, qualitatively, the titration of a strong acid with a strong base.
A. Select HBr and KOH. Do not change the default values in the Experimental Conditions panel. Observe
that the solution in the Erlenmeyer flask is yellow. This color results from the indicator that the
ChemScope adds to the flask.
B. Click the stream button a couple of times to dispense KOH from the buret into the Erlenmeyer flask
containing HBr. Notice that the final volume of KOH in the Experimental Conditions panel and the
titration curve in the Data Window update.
C. HBr (hydrobromic acid) is a strong acid; KOH (potassium hydroxide) is a strong base. What is
happening, chemically, as these two substances are combined?
1. Write the complete balanced equation, ionic equation, and net ionic equation for the titration
reaction that occurs between hydrobromic acid and potassium hydroxide.
2. Look at the net ionic equation you just wrote. As KOH is added to the flask, the molar amount of
OH- it supplies will react with a(n) (greater, lesser, equal) molar quantity of H+. Explain your
choice.
D. Given the net ionic equation you just wrote, you can see that adding KOH to the flask causes hydroxide
ion to react with hydrogen ion.
E. Click the stream button, repeatedly, to continue the titration process. Stop adding KOH after ~ 12.0 mL
KOH have been dispensed.
F. Based on your observations and the information displayed by the ChemScope, answer the following
questions. Toggle between the titration curve and the data table in the Data Window as needed.
3. What happens to the pH of the flask solution as you continue to add KOH? Why?
4. What is the significance of the equivalence point noted on the titration curve? What is the pH at
the equivalence point for a strong acid/strong base titration?
5. In Table 1, list the ions (H+, OH-, none) that are present in the flask during each segment of the
titration.
6. Before the equivalence point, why are no OH- ions present in the flask?
7. After the equivalence point, why are OH- ions present in the flask?
8. On Titration Curve 1, sketch the titration curve, labeling the equivalence point, the region of the
curve where excess H+ ions are present, and the region of the curve where excess OH- ions are
present.
You have examined, qualitatively, the titration of a strong acid with a strong base. Now you will use the
ChemScope to look at such a titration quantitatively.
G. Select HBr and KOH. Change the volume of HBr to 50.0 mL and the concentration of KOH to 0.0877 M.
The ChemScope has selected an appropriate concentration for HBr, which you will calculate when you
complete the Analysis & Conclusions section of the Lab Manual.
9. Record the initial conditions in the appropriate sections of Table 2.
H. Now begin the titration process. Use the stream/drop buttons to add KOH until the flask solution just
turns completely purple.
10. Record your results in Table 2.
Part 2: Titration of Weak Acid with Strong Base
In this type of titration, a strong base is added to a weak acid.
I.
Select HC4H7O3 and NaOH. Do not change the default values in the Experimental Conditions panel.
Since - hydroxybutyric acid (HC4H7O3) is a weak acid, its pKa value is displayed. Observe that the
solution in the Erlenmeyer flask is yellow. This color results from the indicator that the ChemScope
adds to the flask.
J. Before the titration begins, the flask contains the indicator and the weak acid, HC4H7O3, in water. As a
weak acid, - hydroxybutyric acid dissociates in water to some extent.
11. Write the equation for the dissociation of HC4H7O3 in water.
K. Click the stream button a couple of times to dispense NaOH from the buret into the Erlenmeyer flask
containing HC4H7O3. Notice that the final volume of NaOH in the Experimental Conditions panel and the
titration curve in the Data Window update. What is happening, chemically, as these two substances are
combined?
12. Write the complete balanced equation, ionic equation, and net ionic equation for the titration
reaction that occurs between - hydroxybutyric acid and sodium hydroxide.
L. Given the net ionic equation you just wrote, you can see that adding NaOH to the buret causes
hydroxide ion to react with hydrogen ion from the weak acid.
M. Click the stream button, repeatedly, to continue the titration process. Stop adding NaOH once the flask
solution has turned completely red.
N. Based on your observations and the information displayed by the ChemScope, answer the following
questions. Toggle between the titration curve and the data table as needed.
13. What happens to the pH of the flask solution as you continue to add NaOH? Why?
14. Before the equivalence point, why are no OH- ions present in the flask?
15. At the equivalence point, what has happened to the
- hydroxybutyric acid?
16. What is the pH at the equivalence point for this titration?
17. At the equivalence point, the flask contains a solution of C4H7O3-, which is a weak base. When a
weak acid is titrated with a strong base, will the pH at the equivalence point ever be less than or
equal to 7.00? Explain.
18. After the equivalence point, why are OH- ions present in the flask? (Refer to the titration curve or
data table.)
19. On Titration Curve 2, sketch the titration curve and label the equivalence point. On the curve,
find the point where the pH is equal to the pKa for HC4H7O3 and label it.
O. Select HC4H7O3and NaOH. Change the volume of HC4H7O3 to 30.0 mL and the concentration of NaOH
to 0.0980 M. The ChemScope has selected an appropriate concentration for HC4H7O3, which you will
calculate when you complete the Analysis & Conclusions section of the Lab Manual.
20. Record the initial conditions in the appropriate sections of Table 3.
P. Now begin the titration process. Use the stream/drop buttons to add NaOH until the flask solution just
turns completely red.
21. Record your results in Table 3.
Part 3: Titration of Weak Base with Strong Acid
In this type of titration, a strong acid is added to a weak base.
Q. Select NH3 and HCl. Do not change the default values in the Experimental Conditions panel. Ammonia
(NH3) is a weak base; the pKa for its conjugate acid (NH4+) is displayed. Observe that the solution in the
Erlenmeyer flask is purple. This color results from the indicator that the ChemScope adds to the flask.
R. Before the titration begins, the flask contains the indicator and the weak base, NH3, in water. As a weak
base, ammonia reacts with water to some extent.
22. Write the equation for this reaction.
S. Click the stream button a couple of times to dispense HCl from the buret into the Erlenmeyer flask
containing NH3. Notice that the final volume of HCl in the Experimental Conditions panel and the
titration curve in the Data Window update. What is happening, chemically, as these two substances are
combined?
23. Write the complete balanced equation, ionic equation, and net ionic equation for the titration
reaction that occurs between ammonia and hydrochloric acid.
T. Given the net ionic equation you just wrote, you can see that adding HCl to the buret causes hydrogen
ion to react with the weak base.
U. Click the stream button, repeatedly, to continue the titration process. Stop adding HCl once the flask
solution has turned completely yellow.
V. Based on your observations and the information displayed by the ChemScope, answer the following
questions. Toggle between the titration curve and the data table as needed.
24. What happens to the pH of the flask solution as you continue to add HCl? Why?
25. Before the equivalence point, why are no H+ ions present in the flask?
26. At the equivalence point, what has happened to the ammonia? What is the pH at the
equivalence point for this titration?
27. At the equivalence point, the flask contains a solution of NH4+, which is a weak acid. When a
weak base is titrated with a strong acid, will the pH at the equivalence point ever be 7.00 or
greater? Explain.
28. After the equivalence point, why are H+ ions present in the flask?
29. On Titration Curve 3, sketch the titration curve, labeling the equivalence point and the point
where the pH is equal to the pKa for NH4+.
W. Select NH3 and HCl. Change the volume of NH3 to 25.0 mL and the concentration of HCl to 0.107 M.
The ChemScope has selected an appropriate concentration for NH3, which you will calculate when you
complete the Analysis & Conclusions section of the Lab Manual.
30. Record the initial conditions in the appropriate sections of Table 4.
X. Now begin the titration process. Use the stream/drop buttons to add HCl until the flask solution just
turns completely yellow.
31. Record your results in Table 4.
Analysis & Conclusions
Using Titration Data to Determine [Solution]
For all types of titrations, the equivalence point occurs when the molar quantity of H+ is equal to the molar
quantity of OH-. Mathematically, MaVa = MbVb, where M represents molarity and V represents volume.
1. Calculate (below) the unknown molarity for each of the titrations you conducted with the ChemScope.
You recorded this data in Tables 2, 3, and 4.
Interpreting Titration Curves
Now that you've seen the titration curves for the three classes of titrations, you should be able to interpret a
titration curve. Refer to your answers for questions 8, 19, and 29, as needed.
Use Titration Curve 4, below, to answer the next series of questions.
2. What are the pH and volume values at the equivalence point?
3. What class of titration is represented by this titration curve? Explain your answer.
4. The pKa for this acid is 6.15. At what volume does the pH of the solution equal the pKa?
5. What is significant about the volume at which the pH of the solution equals the pKa? (Hint: How does
this volume relate to the equivalence point volume?)
6. Look at the titration curve closely. Where is the solution most resistant to changes in pH?
Use Titration Curve 5 to answer the following questions. Note that this curve, which represents the titration of
20.0 mL of pyridine solution (a weak base) with 0.1520 M HCl (a strong acid) does not provide specific pH
values. For the conjugate acid of pyridine, Ka = 5.90 x 10-6.
7. What is the concentration of the pyridine solution at point A?
8. What is the pH of the solution at point B?
9. Is the pH at the equivalence point less than, equal to, or greater than 7.00?
10. The pH at point D represents the pH of a (weak base, weak acid, strong base, strong acid). Explain
your choice.
Applying What You've Learned
60.0 mL of a 0.0530 M solution of weak acid, HA, are titrated with 0.2120 M NaOH.
The Ka for this acid is 4.2 x 10-6. Use your understanding of the titration process to answer the following
questions.
11. Is the pH at the equivalence point less than, equal to, or greater than 7.00?
12. How many milliliters of NaOH must be added to the HA solution to reach the equivalence point?
13. What is the pH of the solution after 7.5 mL NaOH have been added?