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A Weak Acid – Strong Base Titration
OBJECTIVES
To graphically determine the equivalence point in a weak acid – strong base titration.
To determine the molar concentration of a weak acid solution.
To obtain information about the acid dissociation constant, Ka, of the weak acid being titrated.
INTRODUCTION
An acid-base titration can be monitored using either an acid – base indicator or a pH meter. In either case, the primary
purpose is to determine the equivalence point of the titration. As students have learned previously, the equivalence point
is the point in the titration when the number of moles of added titrant is stoichiometrically equivalent to the original
number of moles of analyte. When an indicator is used in a titration (as used previously), a color change occurs at the
indicator’s endpoint, which coincides with (i.e., occurs at approximately the same pH as) the equivalence point of the
titration. An indicator such as phenolphthalein is commonly used in the titration of a strong acid with a strong base
because the color change occurs when the solution pH is close to 7, which is the pH of the equivalence point.
When a pH meter is used to monitor a titration, the pH of the solution is recorded as the titrant is added. The change in pH
versus volume of added titrant is plotted to give a titration curve for the reaction. In a titration curve, the equivalence
point is observed to occur at the point where very small additions of titrant cause a very rapid increase in pH.
Graphically, the equivalence point can be described as the inflection point in the sharp vertical portion of the plot (where
the slope, ΔpH/ΔV, changes from positive to negative). Figure 1 shows a titration curve for the titration of a strong acid
(HCl) with a strong base (NaOH). The equivalence point is observed to be about midway on the vertical rise.
When a weak acid is titrated with a strong base in aqueous solution, the dissociation of the acid into H3O + (aq) and the
conjugate base of the acid changes the appearance of the titration curve, as well as the information that can be derived
from the curve. Because of the incomplete dissociation of the weak acid, the reaction is in equilibrium, with an acid
dissociation constant (Ka) which is specific to that weak acid.
Figure 2 shows a titration curve for the titration of a weak acid such as acetic acid with a strong base (NaOH). Although
the equivalence point is still observed to be about midway on the sharp vertical rise in pH, several differences are apparent
in the comparison of Figures 1 and 2. There are variations in the initial pH of the analyte, the rate of pH change, and the
pH at the equivalence point. The addition of a strong base to the weak acid creates a build-up of the salt of the weak acid,
producing a buffering effect that resists change in the pH. (Additionally, the pH of the equivalence point corresponds to
the pH of the conjugate base, which also hydrolyzes in water.) These effects are all related to the strength (Ka) of the acid
being titrated.
Figure 1. Titration of 0.1 M HCl with 0.1 M NaOH. Figure 2. Titration of 0.1 M HAc with 0.1 M NaOH.
A Weak Acid-Strong Base Titration Aug18 1
Determination of molar concentration of the acid solution
Regardless of whether the acid being titrated is strong or weak, the moles of NaOH consumed at the equivalence point
equal the volume of NaOH dispensed at the equivalence point times it molar concentration:
moles NaOH (mol) = volume (L) x molar concentration (M)
Eqn. 1
For a monoprotic acid, HA, the balanced equation reveals that one mole of OH – neutralizes one mole of acid:
HA(aq) + OH–(aq) H2O(l) + A–(aq)
Eqn. 2
Consequently, the molar concentration of the acid is determined by dividing the moles of acid neutralized by the volume
(in L) of the acid sample:
mol HA
molar concentration HA (M) = vol HA (L)
Eqn. 3
For polyprotic acids there are multiple dissociation steps and equivalence points, one for each acidic hydrogen present.
The dissociation reactions of a diprotic acid, H2A, are shown below along with the neutralization reactions that occur in a
titration by a strong base.
H2A(aq) + H2O(l) ⇌ H3O+(aq) + HA–(aq)
H2A(aq) + OH–(aq) H2O(l) + HA–(aq)
Eqn. 4a
HA–(aq)+ H2O(l) ⇌ H3O+(aq) + A2- (aq)
HA–(aq)+ OH–(aq) H2O(l) + A2- (aq)
Eqn. 4b
Determination of pKa and Ka of a weak monoprotic acid
For the dissociation of any weak acid, HA:
HA(aq) + H2O(l) ⇌ H3O+(aq) + A–(aq)
Eqn. 5
At equilibrium, the acid dissociation constant, Ka, is described as:
=
[ + ][ − ]
Eqn. 6
[ ]
This can be rearranged to solve for [H3O+]:
[ + ] =
[ ]
Eqn. 7
[ − ]
Using the definition of pH, this equation is rearranged as follows:
[ − ]
[ ]
= − [ + ] = − − ( [ −] ) , = + ([ ])
Eqn. 8
This last expression is known as the Henderson-Hasselbach equation. It can be used to calculate the pKa (and Ka) of a
weak acid. At the equivalence point, the volume of base added is sufficient to neutralize all the acid. At one-half of this
volume of added base, called the half-equivalence point, enough base has been added to neutralize half of the acid. Since
half of the acid reacted to form A-(aq), the concentrations of A-(aq) and HA(aq) are identical. Therefore, at the halfequivalence point, the pH is equal to the pKa.
[ − ]
Since ([ ]) = ( ) = , it follows that
pH = pKa
Eqn. 9
In this experiment, students will conduct a complete weak acid – strong base titration to determine the molar
concentration (molarity) of a solution of acetic acid (CH3COOH) using a previously standardized NaOH solution as the
titrant. A pH meter (LabQuest with pH sensor) will be used to collect pH vs. volume data to create the titration curve from
which the equivalence point can be derived.
Once the equivalence point of the titration is determined, the pH at the half-equivalence point, which is the pKa of acetic
acid, will be determined. The experimental pKa will be compared to the published pKa for acetic acid.
A Weak Acid-Strong Base Titration Aug18 2
Experiment Procedure
MATERIALS: LabQuest with pH Sensor, microstirrer attachment, ring stand, magnetic stirrer, 50-mL buret, buret
clamp(s), small utility clamp, 250-mL beaker, 10-mL graduated cylinder, 100-mL graduated cylinder,
pH 4 and pH 7 buffer solutions, two small beakers for buffer solutions, distilled water bottle, rinse/waste
beaker, personally-owned flash drive.
1. Check out LabQuest PDA, pH Sensor, and
microstirrer from instructor.
2. If necessary, open the LabQuest App folder from the
LabQuest Home Page. To discard previous student
data, tap the Data icon in the upper right corner;
from the Table dropdown menu select “Clear All
Data”. Connect the pH sensor (with attached storage
bottle) to LabQuest Channel 1.
3. Obtain ring stand, buret clamp(s), magnetic stirrer,
and small utility clamp.
4. Obtain a 50-mL buret and a bottle of previously
standardized 0.1 M NaOH. Record the precise
molarity of the NaOH solution on the Report Sheet.
Rinse buret with two 5-mL samples of distilled
water, followed by two 5-mL samples of NaOH. Fill
the buret with NaOH, remove air bubble from tip,
and refill to a volume of 0.00 mL. Secure buret in
buret clamp. Use the utility clamp (or second buret
clamp) to suspend the pH sensor from the ring stand,
as shown in Figure 3.
5. If the pH Sensor is to be calibrated, continue to Step
6. Otherwise, skip to Step 15.
Calibrate the pH Sensor – Optional (Check with Instructor)
6. If the pH Sensor is already connected to the
LabQuest, the pH reading will be displayed. From
the Sensors menu, Choose Calibrate ⩥ CH1 : pH,
and tap Calibrate Now.
7. Remove the storage bottle and lid from the pH
Sensor. DO NOT dispose of or spill the storage
solution – the pH Sensor must be returned to the
storage solution at the conclusion of the experiment.
8. Rinse the tip of the Sensor with distilled water,
catching rinse in waste beaker. Pat Sensor dry with a
Kimwipe. Place the pH Sensor in bottle containing
pH 4 buffer so that the glass bulb at the tip is
immersed.
13. The pH Sensor is now calibrated and ready for use.
Always rinse the pH Sensor with distilled water and
pat dry prior to immersing in a new solution. Note:
When not in use for more than 2 – 3 minutes, the pH
Sensor must be immersed in its original storage
solution.
14. Do not dispose of the pH 4 and 7 buffer solutions –
simply cap the bottles and put away.
Figure 3. Titration set-up.
(Buret clamp or
small utility clamp)
9. Enter the pH of the buffer solution (4) as the known
value for Value 1. When the voltage stabilizes, tap
Keep.
10. Remove the pH Sensor from the pH 4 buffer and
rinse the tip of the sensor with distilled water,
catching rinse in waste beaker. Pat Sensor dry with a
Kimwipe. Place the pH Sensor in bottle containing
pH 7 buffer so that the glass bulb at the tip is
immersed.
11. In the Value 2 field, enter the pH of the second
buffer solution (7). When the voltage reading
stabilizes, tap Keep.
(Second buret clamp
or small utility clamp)
LabQuest
12. Tap OK to complete the calibration process.
A Weak Acid-Strong Base Titration Aug18 3
Titration to Determine Molar Concentration and pKa of a Weak Acid Solution
15. Add 60 mL of distilled water to a clean 250-mL
beaker. Obtain ~15 mL of an acetic acid solution of
unknown concentration in a small, clean, labeled
beaker, then use a clean 10-mL graduated cylinder to
transfer 10.00 mL of the acetic acid to the 250-mL
beaker. Refill the 10-mL cylinder with distilled
water and add to the 250-mL beaker (~80 mL total).
16. Remove the pH Sensor from the storage solution.
Rinse the tip of the Sensor with distilled water,
catching the rinse in a waste beaker; pat dry with a
Kimwipe. Attach the Microstirrer to the bottom of
the pH Sensor. Rotate the paddle wheel of the
Microstirrer to make sure that it does not touch the
bulb of the pH Sensor.
17. Place the magnetic stirrer on the base of the ring
stand. Place the 250-mL beaker containing the acetic
acid solution on the magnetic stirrer. Adjust the
utility clamp so that the bulb of the pH Sensor is
centered and immersed in the acid solution. Adjust
the buret clamp so that the tip of the buret is inside
and slightly below the rim of the beaker (see Figure
3). Turn the magnetic stirrer to the lowest speed.
f. Continue adding NaOH solution until the pH value
is approximately 11.5.
20.
Stop data collection by tapping the red stop icon in
bottom left corner. The graph of pH vs. volume
should immediately be displayed.
21.
Obtain instructor’s approval of data/graph. If the
graph is not sufficiently detailed, a second run will
be required (repeat Steps 15 – 20).
22.
If approved, store the data from the run by tapping
the File Cabinet icon. Remove microstirrer, rinse the
pH Sensor with distilled water, pat dry, and replace
storage solution on pH Sensor. Secure lid firmly to
minimize spillage or evaporation of the storage
solution.
23.
Examine the approved titration data to identify the
volume region where the pH made the greatest
increase. The equivalence point is midway in this
region.
a. Examine the data pairs on the displayed graph by
selecting any data point. As you move the “examine
line”, the pH and volume values of each data point
are displayed to the right of the graph.
b. Identify the equivalence point volume as precisely
as possible and record this value on the Report
Sheet.
An alternate way of determining the equivalence
point of the titration is to take the first derivative of
the pH-volume data:
a. Tap the Table tab and choose New Calculated
Column from the Table menu.
b. Enter d1 as the Calculated Column Name. Select
the equation 1st Derivative (Y, X). Use Volume as
the Column for X and pH as the Column for Y.
Select OK.
c. On the displayed plot of d1 vs. volume, examine
the graph to determine the volume at the peak value
of the first derivative. Record this value on the
Report Sheet.
18. On the LabQuest Meter screen, tap Mode. Change
the data-collection mode to Events with Entry.
Enter Name (Volume), Unit (mL) and select OK.
19. Conduct the titration carefully, as described below.
Note: The titration process is tedious – be patient
and follow instructions!
a. Start data collection by tapping the green arrow in
the bottom left corner.
b. Before adding any NaOH solution, tap Keep and
enter 0 as the initial buret volume in mL. Select OK
to store the first data pair.
c. Add the first increment of NaOH titrant, just
enough to increase the pH about 0.15 units. When
the pH stabilizes, tap Keep and enter the current
buret reading to the hundredths place. Select OK to
save the second data pair.
d. Continue adding NaOH solution in small
increments that raise the pH by about 0.15 units, and
enter the buret reading after each increment. When a
pH value of approximately 5.5 is reached, change to
a 5-drop increment, then a 2-drop increment as the
pH rises more quickly. Enter a new buret reading
after each increment.
e. After a pH value of approximately 10.5 is reached,
again add larger increments that increase the pH by
about 0.15 pH units, and enter the buret level after
each increment.
24.
Optional: To delete the d1 column, tap the Table tab
and choose Delete Data Column d1 from the Table
menu.
25.
To export the data from Run 1 (or Runs 1 & 2, if
directed) as a text file to a flash drive:
a. Insert the flash drive directly into the USB port on
the LabQuest unit, then select Export from the File
menu.
b. Tap on the USB flash drive icon, then tap on the
name field and enter the file name. Tap Done to
A Weak Acid-Strong Base Titration Aug18 4
return to the Export screen, then tap OK to export
the file as a text (.txt) file.
26. Prior to discarding data on LabQuest, (see Step 2),
check flash drive to confirm that text file is present
and not corrupted.
27. Cleanup and Disposal: Collect all waste solutions
in large waste beaker. Test pH, and if needed,
neutralize solution before disposal in lab sink with
plenty of running water. Rinse glassware with tap
water and return to original storage. Return
LabQuest PDA, pH Sensor, and microstirrer to
instructor.
Data Analysis
1. To open the exported .txt file in a spreadsheet
program such as Excel, confirm the program’s file
browser is set to look for all file types, and select
your text file. (In Excel, the Text Import Wizard will
guide the user through a series of questions.
Continue to tap “Next” until the import process is
complete.) Using Excel, manipulate the data to plot
pH vs. Volume for the titration curve. Label the
axes, title the graph, and print the graph. (If a second
titration (Run 2) was necessary, prepare and print a
separate titration curve for Run 2 as well.)
2. The equivalence point is determined from the
printed graph by drawing a vertical line through the
inflection point in the sharp vertical portion of the
plot, about midway on the vertical rise (see Figure
2). The volume of NaOH at the equivalence point is
read directly from the horizontal axis; record this
value on the Report Sheet.
3. The average volume of NaOH is determined from
all three analysis methods (LabQuest data, LabQuest
1st derivative calculation, and printed graph).
4. The molar concentration of the acid is calculated as
described in Eqns. 1 – 3.
5. The volume of NaOH at the half-equivalence point
is determined directly from the average volume of
NaOH at the equivalence point. The pH at the halfequivalence point is read directly from the printed
graph.
A Weak Acid-Strong Base Titration Aug18 5
A Weak Acid – Strong Base Titration
Prelaboratory Assignment
Date: __________
Name: ______________________________ Partner(s) Name:__________________________
1. Write the net ionic equation for the reaction of aqueous solutions of weak acid CH3COOH with strong base NaOH.
2. For a weak acid (e.g., CH3COOH) that is titrated with a strong base, (e.g., NaOH), what species (ions and/or
molecules) are present in the solution at the equivalence point?
3. For a weak acid (e.g., CH3COOH) that is titrated with a strong base (e.g., NaOH), what species (ions and/or
molecules) are present in the solution at the half-equivalence point?
4. For this experiment:
a. Is a personal flash drive required? _______
b. How many titrations (“runs”) are required?
c. How will students obtain the molarity of the NaOH(aq)?
d. Is an indicator used in this experiment? If “Yes”, identify the indicator and the pH range over which it can be
used. If “No”, explain why no indicator is needed.
5. a. A 12.75 mL volume of 0.0998 M NaOH was used to titrate 10.00 mL of a weak monoprotic acid solution to its
equivalence point. Determine the molar concentration of the weak acid solution.
b. If the weak acid in question 5a above was diprotic, calculate the molar concentration of the weak acid solution.
A Weak Acid-Strong Base Titration Aug18 6
A Weak Acid – Strong Base Titration
Report Sheet
Date: __________
Name: ______________________________ Partner(s) Name:__________________________
Volume of CH3COOH (mL):
______________
Initial Volume of NaOH (mL):
Sample Code:
Temperature: _________
Molarity of NaOH (from label): ______________
_______
RUN 1
From LabQuest
Titration Curve
From LabQuest
1st Derivative
From Printed
Titration Curve
0.00
Instructor’s
Approval of
LabQuest Data
Volume of NaOH at
Equivalence Pt (mL)
Include printed
graph of titration
curve with
Report Sheet.
Average Volume of
NaOH at Equivalence Pt
RUN 2 (If necessary)
From LabQuest
Titration Curve
From LabQuest
1st Derivative
From Printed
Titration Curve
Instructor’s
Approval of
LabQuest Data
Volume of NaOH at
Equivalence Pt (mL)
Average Volume of
NaOH at Equivalence Pt
Moles of NaOH used at equivalence point:
_______________
Moles of CH3COOH consumed at equivalence point: ________________
Molar Concentration of CH3COOH(aq):
________________
Show detailed calculation:
Volume of NaOH at Half-Equivalence Point (mL):
________________
pH of CH3COOH(aq) at Half-Equivalence Point:
________________
Ka of CH3COOH(aq): __________________
Show detailed calculation for Ka:
A Weak Acid-Strong Base Titration Aug18 7
A Weak Acid – Strong Base Titration
Post-lab Questions
Date: __________
Name: ______________________________ Partner(s) Name:__________________________
1. Look up the Ka value for acetic acid from a reliable source, noting the temperature at which the Ka value was
recorded. Compare the published value with your experimental value, and calculate the percent error. Give a plausible
reason for any discrepancy.
2. If during the calibration process, the pH Sensor was mistakenly calibrated to be 1.0 pH unit higher than each buffer,
how would this affect your calculation of:
a. The molar concentration of the weak acid – too high, too low, or unaffected? Explain your answer.
b. The Ka of the weak acid – too high, too low, or unaffected? Explain your answer.
3. If prior to the titration, the initial volume of NaOH was recorded as 5.50 mL instead of 0.00 mL, how would this
affect your calculation of:
a. The molar concentration of the weak acid – too high, too low, or unaffected? Explain your answer.
b. The Ka of the weak acid – too high, too low, or unaffected? Explain your answer.
4. Explain why it is good technique to slow the addition of NaOH titrant near the equivalence point, as well as add
NaOH titrant beyond the equivalence point.
A Weak Acid-Strong Base Titration Aug18 8
Vernier Format 2
weak acid-strong base titration 11-1.txt 7/25/2022 17:52:21
Run 1
Volume
pH
V
p
mL
0
0,95
1,49
1,95
3
4,32
5,59
7,1
8,21
9,55
10,95
12,09
13,11
13,32
13,53
13,78
14
14,2
14,41
14,62
14,85
15,09
15,31
15,51
15,72
15,92
16,15
16,34
16,59
16,8
17,01
17,23
17,38
17,6
17,68
17,72
17,81
17,95
18,01
18,1
3,19
3,42
3,77
3,89
4,1
4,3
4,47
4,64
4,78
4,95
5,14
5,34
5,58
5,64
5,69
5,74
5,82
5,88
5,95
6,03
6,11
6,19
6,28
6,36
6,46
6,54
6,66
6,78
6,87
7,08
7,26
7,62
7,8
8,17
8,64
8,89
9,02
9,11
9,19
9,25
18,19
9,3
18,28
9,37
18,38
9,4
18,47
9,44
18,55
9,47
18,61
9,51
18,7
9,52
18,8
9,58
18,9
9,61
19
9,65
19,09
9,67
19,19
9,7
19,25
9,73
19,32
9,76
19,4
9,79
19,5
9,81
19,65
9,84
19,75
9,88
19,81
9,89
21
9,94
Vernier Format 2
weak acid-strong base titration 11-1.txt 7/25/2022 17:52:21
Run 2
Volume
pH
V
p
mL
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