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Lab 9: Acid-Base Equilibria, Part II
Report Form – for on campus students
Due Mon. Apr. 5 at 11:59 PM. Upload .pdf (preferred) or MS Word file to
Brightspace.
Group Member Names:
Question #1. (0.5 pt.) In your own words, describe the goal(s) of this experiment.
TITRATING A WEAK BASE WITH A STRONG ACID
Question #2. (0.5 pt.) Describe the preparation (dilution) of the NH3 solution that was used for
the titration.
Question #3. (0.5 pt.) Describe the procedure that you used for the titration of NH3 with HCl;
that is, write an overview of the procedure that was used.
1
Question #4. (0.5 pt.) Write a balanced chemical equation for the NH3-HCl titration reaction.
Question #5. (0.5 pt.) Complete Table 1 below using the data from the NH3-HCl titrations.
(Use appropriate significant figures.)
Concentration of HCl standard solution = _____________________ M.
Report Table 1 Volumes of NH3 and HCl Used for Titrations.
titration method
titration
type
colorimetric
careful
potentiometric
careful
volume of NH3
solution (mL)
volume of HCl required to reach
equivalence point (mL)
Calculation #1. (0.5 pt.) Calculate the concentration of NH3 in the diluted NH3 solution using
the colorimetric titration data.
• Show all work and label units.
Calculation #2. (0.5 pt.) Using your colorimetric titration results, calculate the concentration of
NH3 in the original (undiluted) stock solution.
• Show all work and label units.
2
Question #6. (0.5 pt.) Paste your plot of the second derivative of the potentiometric (pH)
weak base-strong acid titration curve (or the plot that you used to determine the equivalence
point) below.
• Label the equivalence point on the graph.
• Indicate the volume of titrant at the equivalence point in the space below.
Volume of titrant* at equivalence point = ____________
*record volume to 2 decimal places
Calculation #3. (0.5 pt.) Calculate the concentration of NH3 in the diluted NH3 solution using
the pH titration data.
• Show all work and label units.
Calculation #4. (0.5 pt.) Using your pH titration results, calculate the concentration of NH3 in
the original (undiluted) stock solution.
• Show all work and label units.
3
Question #7. (0.5 pt.) Calculate the experimental value of Kb for NH3.
• Include a screenshot of the graph you used to make this determination (i.e. pH vs.
volume of HCl added).
• Label the axes and provide a title.
• Label the half-equivalence point and the equivalence point on the graph to illustrate
how you obtained the value of Kb.
• Show all work and label units.
Question #8. (0.5 pt.) Compare the value of Kb you determined from the potentiometric
titration data with the accepted value of Kb for NH3 (1.8 x 10‒5) and calculate the % error.
%𝑒𝑟𝑟𝑜𝑟 =
|𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 𝑣𝑎𝑙𝑢𝑒 − 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑣𝑎𝑙𝑢𝑒|
𝑥 100
𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑣𝑎𝑙𝑢𝑒
4
TITRATING AN UNKNOWN WEAK ACID WITH
A STRONG BASE
Question #9. (0.5 pt.) Describe the preparation of the unknown weak acid solution that was
used for the titrations, including the exact mass of acid.
Question #10. (0.5 pt.) Describe the procedure that was used for the titration of the unknown
weak acid with NaOH; that is, write an overview of the procedure that was used.
Question #11. (0.5 pt.) Complete Table 2 below using the data from the unknown weak acidNaOH titrations.
• Use appropriate significant figures.
Concentration of NaOH standard solution = _____________________ M.
Report Table 2. Volumes of Unknown Weak Acid and NaOH Used for Titrations.
titration method
titration
type
colorimetric
careful
potentiometric
careful
volume of unknown
volume of NaOH required to
weak acid solution (mL) reach equivalence point (mL)
5
Calculation #5. (0.5 pt.) Calculate the molar concentration of the unknown weak acid by using
the colorimetric titration data. (Include units and show all work.)
Question #12. (0.5 pt.) Paste your plot of the second derivative of the potentiometric (pH)
weak base-strong acid titration curve (or the plot that you used to determine the equivalence
point) below.
• Label the equivalence point on the graph.
• Indicate the volume of titrant at the equivalence point in the space below.
Volume of titrant* at equivalence point = ____________
*record volume to 2 decimal places
Calculation #6. (0.5 pt.) Calculate the molar concentration of the unknown weak acid by using
the potentiometric (pH) titration data.
• Show all work and label units.
6
Question #13. (0.5 pt.) Calculate the experimental value(s) of Ka for the weak acid.
• Include a screenshot of the graph you used to make this determination (i.e. pH vs.
volume of NaOH added).
• Label the axes and provide a title.
• Label the half-equivalence point(s) and the equivalence point(s) on the graph to
illustrate how you obtained the value(s) of Ka.
• Show all work and label units.
Question #14 (0.5 pt.) By using the pH titration data, explain whether the unknown weak acid
is monoprotic, or diprotic.
Question #15 (0.5 pt.) Write a general balanced chemical equation for the titration reaction of
the unknown weak acid (use either HA or H2A as the chemical formula for the acid).
Calculation #7. (0.5 pt.) Calculate the average concentration of unknown weak acid using the
data from the careful colorimetric and potentiometric titrations.
• Show all work and label units.
• Use appropriate significant figures.
Average concentration of unknown weak acid = _______________ M.
7
Calculation #8. (1 pt.) Calculate the molar mass (in g/mol) of the unknown weak acid.
• Show all work and label units.
DISCUSSION
Question #16 (1 pt.) Why is it necessary to use a buffer solution to calibrate a pH electrode?
Why isn’t DI water used? Be specific with your reasons.
Upload your completed report in MS Word or .pdf format to the Lab 9
assignment portal in Brightspace. It is due Mon. Apr. 5 by 11:59 PM.
8
Tech_Lab 9: Acid-Base Equilibria, Part II
(CHM 11600- Spring 2021)
Lab 9 (25 pts.) consists of the following:
1. A pre-lab quiz (found on Brightspace) (10 pts.)
2. A Lab Report (15 pts)
NOTE: Pages 4-17 contain background and general procedures for the
experiment. The specific procedure for this experiment begins on p. 18.
Record your group members’ names.
PRELAB PRACTICE QUESTIONS
As part of your individual preparation for lab, read the experiment and answer the following
questions. (Your answers will not be collected or graded.)
You will take a quiz on Brightspace related to these concepts.
1. Write a balanced chemical equation (including states of matter by using state symbols) for
the weak base-strong acid titration you will perform in lab this week.
2. Write a balanced chemical equation (including states of matter by using state symbols) for
the reaction of an unknown diprotic weak acid, H2A, with sodium hydroxide (NaOH).
3. Explain how you will prepare the ammonia (NH3) stock solution for titration. Be specific in
terms of the volumes and glassware you will use.
4. Explain how you will prepare the solution of unknown acid for titration. Be specific in
terms of the volumes, masses and glassware you will use.
5. For each of the following titrations, state which two buffer solutions will be used to
calibrate the pH electrode and which indicator will be used for the titration.
a. titration of weak base with strong acid
b. titration of unknown weak acid with strong base
1
6. Based on the pH titration curve shown below for the titration of 0.1 M NH 3 with 0.1 M
HCl, determine an experimental value for Kb of NH3. Calculate the % error of the
calculated Kb value as compared to the accepted value (Kb = 1.76 x 10–5)
7. A student titrated a 15.00-mL sample of a solution containing a weak, monoprotic acid
with NaOH. The titration required 17.73 mL of 0.1036 M NaOH to reach the equivalence
point. Calculate the concentration (in M) of the weak acid in the sample.
8. If the sample solution described in the preceding question contained 0.1845 g of the weak
acid, calculate the molar mass (in g/mol) of the weak acid.
2
9. Explain why the pH titration curve shown below for the titration of an unknown weak acid
with a strong base has two inflection points. What are the points A, B and C on the pH
titration curve called? Choose from the following options (you won’t use them all): first
equivalence point, second equivalence point, first half-equivalence point, second halfequivalence point.
Fill in the blanks in the sentences below for each of the points A, B and C. (Choose from
the following options (you won’t use them all): Ka1, Ka2, concentration of HA,
concentration of H2A.
You will use information from point A to calculate ___________________.
You will use information from point B to calculate ___________________.
You will use information from point C to calculate ___________________.
10. Explain why the pH of the solution is greater than 7 at point C in the titration described in
the preceding question.
11. A student titrated a diprotic weak acid (H2A) with 0.100 M NaOH. The titration required
22.58 mL of NaOH solution to reach the equivalence point. How many moles of diprotic
acid were present?
12. In this lab, you will titrate an ammonia (NH3) solution. Review the Material Safety Data
Sheet (MSDS) (S25164.pdf (fishersci.com) for 1 M NH3 in water, which can be found at the
link given below. What personal protective equipment must you wear when handling this
solution? What should you do if you get this solution on your skin? What should you do if
you spill this solution on your clothing?
3
LAB LEARNING OBJECTIVES
•
•
•
•
Determine the concentration of a solution containing an acid (or base) by using a pH
titration curve.
Determine the value of Ka (for a weak acid) by using a pH titration curve.
Determine the value of Kb (for a weak base) by using a pH titration curve.
Determine the molar mass of an unknown acid (or base) by using a pH titration curve.
INTRODUCTION: ACID-BASE TITRATIONS (FROM PART I)
Reactions between acids and bases that are dissolved in water occur almost instantaneously;
they occur nearly as fast as the two solutions can be mixed. These reactions also tend to go to
completion, reacting until all of the limiting reagent is consumed. When exact stoichiometric
amounts of acid and base have been mixed, the reaction is said to have reached the
equivalence point – the point at which all of the acid has reacted with the base, and vice versa.
The technique of slowly adding an acid to a base (or vice versa) until the reaction has reached
the equivalence point is known as a titration. In this lab, you will perform two types of acid–
base titrations: colorimetric titrations and pH (or potentiometric) titrations.
Colorimetric Titrations
One way to determine the equivalence point in a titration is to use a colored indicator to show
when the equivalence point of the reaction has been reached. Indicators take many forms, but
they are often substances whose solutions change color due to changes in pH. Two indicators
with which you may already be familiar are litmus and phenolphthalein. In aqueous solution,
phenolphthalein is colorless when the solution is acidic and pink when the solution is basic.
In theory, the indicator should turn color exactly at the equivalence point. In practice, however,
each indicator has an endpoint—the pH at which it turns color—that might be slightly different
from the pH at the equivalence point of the reaction. Titrations of a strong acid with a strong
base that use phenolphthalein as the indicator, for example, should be stopped just before the
solution turns a permanent pink color. In this way, the endpoint of the indicator (pH 8.3 for
phenolphthalein) is brought as closely as possible to the equivalence point of the reaction (pH 7
for strong acid–strong base titrations).
In this lab, your first colorimetric titration will be a “scout” titration, in which you will add the
titrant continuously at a moderate rate to find the approximate volume required to reach the
4
equivalence point. This “scout volume” provides an estimate, which allows you to conduct
subsequent titrations with better accuracy. The second colorimetric titration will be done more
carefully in order to determine, more accurately, the volume of titrant required to reach the
equivalence point.
pH (Potentiometric) Titrations
In a potentiometric (pH) titration, the equivalence point is determined using a pH titration
curve, which is a plot of pH (y-axis) versus volume of titrant added (x-axis). The equivalence
point can be estimated from the inflection point of the titration curve (see Figure 1 below).
Indicators are not used in potentiometric titrations because they are not needed to identify the
equivalence point (this is done by using graphical analysis).
Figure 1 pH Titration curve for the titration of acetic acid with NaOH.
5
Determining the pKa of a Weak Acid From a Titration Curve
A pH titration curve can also be used to determine the value of the equilibrium constant, Ka, for
a weak acid, HA (see Figure 2 below). At the halfway point between the beginning of the
titration and the equivalence point, half of the acid (HA) has been converted to its conjugate
base (A‒); therefore, the concentration of remaining acid (HA) is equal to the concentration of
conjugate base (A‒) present (i.e., [HA] = [A‒]). At this point (the “1/2 equivalence point”), pH =
pKa, as can be seen when you consider the Ka expression for the weak acid.
𝐾𝑎 =
[𝐻3 𝑂+ ][𝐴− ]
[𝐻𝐴]
At the ½ equivalence point,
[HA] = [A-]
So,
Ka = [H3O+]
And,
pKa = pH
Figure 2 Determining the value of Ka for a weak acid from a pH titration curve.
6
PART II. ANALYSIS OF POLYPROTIC ACIDS AND BASES
In the second part of this activity, you will plan and then carry out titrations to analyze or
characterize some substance(s) that you may not have worked with in the previous labs. You
will use the same techniques as those used in Part I; that is, colorimetric and pH titrations.
In Part II, you will perform
•
•
a weak base-strong acid titration and,
a weak acid (possibly polyprotic)-strong base titration.
Experimental Design
In the first titration sequence, you will titrate a solution of ammonia (NH3; weak base) of
unknown concentration with a standard solution of hydrochloric acid (HCl) and use the titration
data to calculate the concentration of ammonia.
In the second titration sequence, you will titrate a solution of an unknown weak acid with a
standard solution of sodium hydroxide (NaOH) and use the titration data to calculate the
concentration, and molar mass, of the unknown weak acid.
Each group is to complete a minimum of three titrations for each sequence: two colorimetric
(one scout and one careful) and one (careful) pH titration.
Groups should split into two subgroups to collect data as outlined in the tables below. The
subgroups will compare the results obtained from the two titration methods.
Titration of Weak Base with Strong Acid
step
Subgroup A (first buret)
Subgroup B (second buret)
1
Do scout colorimetric titration.
Set up LabQuest 2 & calibrate pH
electrode.
2
Subgroup A provides estimated equivalence point volume from the scout
titration to Subgroup B.
3
Do careful colorimetric titration.
Do pH (potentiometric) titration.
For the other titration the subgroups will switch responsibilities.
7
Titration of Unknown Weak Acid with Strong Base
step
Subgroup A (first buret)
Subgroup B (second buret)
1
Do scout colorimetric titration.
2
Subgroup B provides estimated equivalence point volume from the scout
titration to Subgroup A.
3
Do pH (potentiometric) titration.
Do careful colorimetric titration.
SAFETY
Wear your goggles at all times in the laboratory.
Wear gloves. If you leave the lab, take the gloves off and recycle them so you don’t transport
any hazardous materials that might be on the gloves outside of the lab. Put on new gloves
when you return to lab.
Avoid inhalation of ammonia vapor. Dispense the stock solution of ammonia in the main hood.
Prepare the diluted ammonia solution under an exhaust snorkel.
8
GENERAL PROCEDURES
The general procedures given below describe the process used for both sets of titrations, so
these instructions exclude references to the specific reagents used for the analyte and titrant in
each titration. Be sure to read the specific directions starting on p. 18 before attempting the
titrations. Note, in particular, that the ammonia stock solution must be diluted before
titrating the ammonia with strong acid.
 Review Volumetric Measurement Techniques at Brightspace > Labs > Reference Materials
for detailed information on filling burets and removing air bubbles.
Colorimetric Titrations
• Using a clean, rinsed 25.00-mL pipet, measure exactly 25.00 mL of the solution to be
titrated into a clean Erlenmeyer flask.
•
Add one or two drops of indicator solution to the flask.
•
Either set up a magnetic stir plate or plan to swirl the flask throughout the titration.
•
Support a 50.00-mL buret on a ring stand with a buret clamp. Fill the buret with titrant
before setting it over the Erlenmeyer flask containing the sample. Note: for each new
titration, the buret must be filled with titrant so that the volume reading is near 0.00 mL.
•
Position the buret so that the liquid stream from the buret can be directed into the flask
on a magnetic stir plate, if using one.
•
“Scout” titration: with stirring, add the titrant from the buret about 1 mL at a time to the
sample until the color change persists for 15 – 30 seconds. Record all necessary
measurements.
•
“Careful” titration: repeat with a second 25.00-mL sample, adding the titrant about 1 mL
at a time until you are within 1 – 2 mL of the equivalence point as determined in the
“scout” titration. Then, decrease the addition of the titrant to 2 drops at a time until the
endpoint point is reached.
Choosing an Appropriate Indicator for a Colorimetric Titration
Appropriate indicators must be chosen for each procedure. Indicators are chosen to change
color at, or as close as possible to, the equivalence point of a titration.
Phenolphthalein changes color in the pH 8 – 10 range and works well to signal the equivalence
point in titrations of either strong or weak acids with strong bases.
Methyl red changes color (yellow to pink/red) in the pH 4 – 6 range and works well to signal the
equivalence point in titrations of weak bases with strong acids.
Indicators are not used in potentiometric titrations because they are not needed to identify the
equivalence point (this is done by using graphical analysis).
9
Potentiometric (pH) Titrations
Instrumentation can be a great advance in data collection. However, it seems that any gain in
precision and accuracy is offset by additional time needed to care for, calibrate, and operate
the equipment along with an increase in cost. At the current time, the cost to replace a pH
electrode of the type you will use is $100.
Care and Handling of pH Electrodes
•
•
•
•
Keep the electrode moist (wet) at all times. When you are not using the electrode, it
should be in the storage bottle or tube containing storage solution. Be sure the electrode
is stored properly when you have finished using it.
Be very careful of the tip of the electrode. The electrode has a thin membrane at the
bottom that is easily broken.
Never dry the electrode by rubbing it with a paper towel. Just “dab” water droplets with a
lint-free paper such as a Kimwipe or Accuwipe.
When using a magnetic stirring bar, do not allow it to hit the electrode.
Connect the pH electrode to an iPad
Log in to the iPad and open Turn on the
the Graphical Analysis app
wireless pH
and select the Sensor Data
probe. A red
Collection option.
LED will blink.
Figure 3a.
Figure 4b.
T connect the wireless
probe, you must select
the device matching the
ID found on the back of
the probe.
Figure 4c.
A blinking
green LED
means the
probe is
connected. Tap
DONE.
Figure 4d.
10
Calibration of pH Electrode
You must calibrate the equipment for a pH titration. As you go through the calibration
procedure, the display will not show a reasonable pH value that corresponds to the pH of the
buffer solutions until both buffers have been used for the calibration process.
You only need to recalibrate the equipment if you need a different pH range for a titration, or if
you turn off the equipment.
Calibration pH buffers are provided in small jars.
•
•
•
Use the buffer solution in the jar provided.
Do not transfer the buffer solution to another container.
Do not discard the buffer solution.
Choosing Appropriate Buffer Solutions to Calibrate the pH probe for the
Potentiometric Titrations
To determine the pKa of the species being analyzed from the buffer region of the titration, the
volume at the half-equivalence point needs to be measured accurately. The goal is to calibrate
the pH probe for the buffer region of the titration so that an accurate measurement of the pH
at the half-equivalence point can be obtained.
You will use 2 buffer solutions to calibrate the pH probe for each type of the titration (i.e., the
weak base-strong acid titration, and the unknown weak acid-strong base titration.
For each type of titration, you should select one buffer solution with a pH more acidic than the
expected pH at the half-equivalence point and one buffer solution with a pH more basic than
the expected pH at the half-equivalence point.
An example of a two-point calibration using pH 4 and pH 7 buffer solutions follows.
You will need jars of pH 4 and pH 7 buffer solutions for a two-point calibration of the electrode.
Use the buffer solutions in the jars provided. Do not transfer the buffer solutions to another
container. Do not discard the buffer solutions.
11
•
Tap the “pH:” button at the bottom right of the screen, then Calibrate.
•
The pH electrode is kept in a bottle of storage solution. Loosen the cap on the storage
solution bottle to be able to remove the electrode.
•
Support the pH probe by securing it to a ring stand with a utility clamp.
•
Rinse the electrode with DI water (use a beaker to collect the rinses), blot it dry with a
Kimwipe and gently put the electrode into the jar containing the pH 4 buffer solution.
•
Swirl the jar gently for about 15 seconds and then enter”4.00″ for the first known value.
Tap KEEP to store the calibration point.
•
Remove the electrode from the pH 4 buffer, rinse it with DI water and blot dry. Place the
electrode in the jar containing the pH 7 buffer solution.
•
Swirl the jar gently for about 15 seconds and then enter “7.00” for the second known
value. Tap KEEP to store the calibration point.
•
Tap APPLY. Your pH electrode is now calibrated and ready to measure the pH of
solutions.
•
Remove the pH probe from the pH 7 buffer, rinse with DI water, and blot dry.
•
Place the pH probe in DI water or in the bottle with storage solution.
12
Collecting pH (Potentiometric) Titration Data
Figure 4 Experimental apparatus for potentiometric (pH) titrations.
13
•
Pipet 25.00 mL of the solution to be titrated into a clean 100-mL beaker containing a
magnetic stir bar. Add enough DI water (about 15 mL) so that the tip of the pH
electrode is immersed and far enough above the magnetic stir bar that the stir bar
does not hit the electrode as the bar spins. Set the beaker on a magnetic stir plate.
•
Support a 50.00-mL buret on a ring stand. Fill the buret with titrant before setting it
over the beaker containing the solution to be titrated. For each new titration, the
buret must be filled with titrant so that the volume reading is near 0.00 mL.
 Review Volumetric Measurement Techniques at Brightspace > Labs > Reference Materials
for detailed information on filling burets and removing air bubbles.
•
Adjust the 50.00-mL buret filled with titrant so that the stream from the buret can
be directed into the beaker on the magnetic stir plate (see Figure 4 above).
•
Adjust the stirring rate to a moderate rotation speed. A stir rate between 5 and 6 is
usually sufficient.
Set the Data Collection Parameters
The default setting for the Graphical Analysis app is to collect data over time, but for a titration,
you will want to collect a pH value each time a specific volume of titrant is added.
To change the settings, tap the “Mode:” button on the bottom tool bar. From the Mode menu,
select Event Based.
Events with Entry should be selected.
Enter the following in the corresponding fields:
Event Name: mL NaOH
Units: mL
Tap DONE
14
Now each time a pH value is recorded, you will be prompted to enter the total volume of titrant
dispensed (= current buret reading – initial buret reading). Record the initial buret reading.
1. Obtain the equivalence point of the colorimetric scout titration performed by your
subgroup. This is the approximate equivalence point for your potentiometric titration.
2. When ready to start collecting data, tap COLLECT.
3. When the pH stabilizes, click KEEP. You will be prompted to enter the cumulative volume
of titrant that has been dispensed by the buret, which at this point is 0.00. Tap KEEP
POINT.
4. Begin the titration by adding the NaOH solution of unknown concentration to the HCl
solution about 1 mL at a time until you are within 1-2 mL of the predicted equivalence
point (see Step #1 above). After each addition, wait for the pH to stabilize, and then click
KEEP and enter the total cumulative volume dispensed by the buret.
If you erroneously click KEEP before the pH has stabilized, or make a mistake while
entering the volume, you can edit the entry after you are finished with the titration. You
must press STOP to edit entries.
If you stop the data collection prematurely, click COLLECT again and choose APPEND to
continue recording data.
4. Continue adding titrant 2 mL at a time until you are within 1-2 mL of the predicted
equivalence point, and then decrease the addition of base to 2 drops at a time. Wait only
10-15 seconds for the pH to stabilize before selecting KEEP and entering the volume after
each addition. Prolonged stirring introduces CO2 from the air and will change the pH of
the solution.
5. When you are 1 mL past the equivalence point, return to adding NaOH about 2 mL at a
time until you are approximately 10 mL beyond the equivalence point.
6. When you have finished the titration, press STOP.
7. Rinse the electrode with DI water and immerse it in the bottle of storage solution. Pour
the titrated solution down the drain. BE CAREFUL! Do not drop the magnetic stirring bar
down the drain. Rinse the magnetic stirring bar with DI water for reuse.
15
Saving Your Data
To save your data, tap on the File icon in the upper left-hand corner.
1. If you will analyze your data in the lab using the Graphical Analysis app on the iPad, tap on
Save As. Give your data an appropriate file name and save in the desired destination.
2. If you will analyze your data away from lab, tap on Export and select Comma-Separated
values (.CSV). Give your data an appropriate file name and save to the desired
destination. This will allow you to open your data with Excel or some other spreadsheet
program.
Make sure that any data you need outside of lab, including data for the first or second
derivative plots, are exported rather than saved.
Determining Equivalence Point from Derivative Curve
You will determine the equivalence point for the titration using graphical analysis. First, you
must calculate and graph the second derivative.
•
It is easier to interpret where the second derivative crosses the x-axis when the data is
represented as a line rather than points. Tap the Graph tools icon.
•
Choose Edit Graph Options and select Lines for Appearance.
•
In your data set table, tap on the three dots (“…”) next to your pH column heading and
select Add Calculated Column.
•
Name the column “2nd Derivative”.
•
Tap INSERT EXPRESSION and choose 2nd Derivative (Y,X) and then APPLY. (Y = pH, X =
volume)
•
Adjust the y-axis of your graph to display the second derivative data and to not display the
pH data. Tap the Autoscale button. Your graph should now display the second derivative
of your titration curve, similar to Figure 6 below.
16
Figure 5 Second derivative of a pH titration curve.
•
Zoom in on the area where the second derivative crosses the x-axis. Tap the Graph tools
icon in the lower left of your screen and turn on “Interpolate.” You can now tap
anywhere on your graph and a vertical line will appear with its x-value noted on the x-axis.
Move the line to where the graph intersects the x-axis and estimate the volume of NaOH
added at the equivalence point. Record the volume of NaOH added to within two places
beyond the decimal.
•
Print a graph showing how you determined the equivalence point from the pH titration
data. Identify and label (by hand) the equivalence point and volume of titrant at the
equivalence point on your printed graph.
17
TITRATING A WEAK BASE WITH A STRONG ACID
Goals: your group will design and carry out procedures to
•
•
•
titrate 25.00-mL samples of the diluted weak base, NH3, in two ways (colorimetric and pH)
calculate and compare the concentration of the diluted weak base by using data from
both titration methods
determine the value of pKb from the pH titration data, and then determine an
experimental value for Kb for NH3, and
NH3(aq) + H2O(l) ⇌ NH4+(aq) + OH‒(aq)
[𝑁𝐻4 + ][𝑂𝐻 − ]
𝐾𝑏 =
[𝑁𝐻3 ]
•
calculate the concentration of the original NH3 stock solution.
Procedure
Equipment Used
• pH probe (1); front benchtop (in plastic pitcher)
• 50-mL buret (2); lower cabinet
• 10-mL volumetric pipet (1); lower cabinet
• 25-mL volumetric pipet (2); lower cabinet
• 100-mL beaker (3); student drawer
• 250-mL beaker (2); student drawer
• metal spatula (1); student drawer
• stir bar (2); student drawer
• magnetic stirrer/hotplate (2); table
• ring stand (3); table
• buret clamp (2); table drawer
• utility clamp (1); table drawer
• 125-mL Erlenmeyer flask (4); upper cabinet
• 100-mL volumetric flask (2); upper cabinet
Reagents Used
•
•
•
•
ammonia (NH3) stock solution
HCl standard solution
phenolphthalein or methyl red indicator solutions
pH 4, 7 and 10 buffer solutions
18
Preparing a Diluted Ammonia Solution for Titration
Avoid inhalation of ammonia vapor. Dispense the stock solution of ammonia in the main
hood. Prepare the diluted ammonia solution under an exhaust snorkel.
Because the stock solution of ammonia is too concentrated to titrate directly, you will need to
prepare a diluted solution. Prepare a diluted solution as follows.
•
Use a beaker to obtain about 30 mL of the ammonia stock solution.
•
Use a 10.00-mL volumetric pipet to transfer 10.00 mL of ammonia stock solution from the
beaker to a 100.00-mL volumetric flask.
•
Add some DI water to the flask and mix well, then carefully fill the flask to the mark with
DI water (see Figure 7 below) and mix well again. (To mix, cover the mouth of the flask
with a small square of Parafilm and invert several times.)
Figure 6 Filling a 100-mL volumetric flask to the mark.
19
Titrating the Diluted Ammonia Solution with HCl
Each group has been allotted 150 mL of HCl standard solution to perform the (2) colorimetric
and (1) potentiometric titrations of the diluted ammonia solution.
â–º Follow the general procedures described in the preceding sections on pp. 9-17.
Record the following:
1. Concentration of the HCl standard solution
2. Volume of HCl and NH3 solutions used in the scout and careful colorimetric
titration
3. Volume of NH3 solution used in the potentiometric (pH) titration
4. The screenshot of the potentiometric (pH) curve (pH vs. volume of HCl added)
used to determine the experimental value(s) of Kb. Label the axes and provide a
title. Label the half-equivalence point and the equivalence point on the graph to
illustrate how you obtained the value of Kb.
5. The screenshot of the potentiometric (pH) curve (the 2nd derivative) used to
determine the equivalence point. Identify and label the equivalence point.
Record the volume of titrant at the equivalence point.
20
TITRATING AN UNKNOWN WEAK ACID WITH
A STRONG BASE
Goals: your group will design and carry out procedures to
•
titrate 25.00-mL samples of the unknown weak acid solution in two ways (colorimetric
and pH),
determine whether or not the unknown acid is monoprotic (HA), or diprotic (H2A),
•
Monoprotic:
HA + H2O ⇌ H3O+ + A-
Ka
H2A + H2O ⇌ H3O+ + HA[𝐻3 𝑂+ ][𝐻𝐴− ]
𝐾𝑎1 =
[𝐻2 𝐴]
Ka1
HA- + H2O ⇌ H3O+ + A2[𝐻3 𝑂+ ][𝐴2− ]
𝐾𝑎2 =
[𝐻𝐴− ]
Ka2
Diprotic:
•
calculate and compare the concentration of the unknown weak acid by using data from
both titration methods
•
calculate the molar mass of the unknown acid
•
determine the Ka value(s) for the acid; that is, Ka if the acid is monoprotic, Ka1 and Ka2 if
the acid is diprotic.
pH Titration curves for diprotic acids show two sigmoidal regions if the two Ka values (Ka1 and
Ka2) differ by several orders of magnitude (Figure 8).
21
Figure 7 pH Titration curve of a diprotic acid with a strong base.
In this lab, you can assume that the color change for the colorimetric titration would occur near
the second equivalence point for the weak acid. Carry out the pH titration of the unknown
weak acid using small increments of base when within 1 – 2 mL of both one half of the end
point volume, and the end point volume from the colorimetric titration in order to avoid adding
too much base and possibly missing the first equivalence point should the acid be diprotic.
Procedure
Equipment Used
•
•
•
•
•
•
•
•
•
•
•
•
•
•
pH probe (1); front benchtop (in plastic pitcher)
50-mL buret (2); lower cabinet
10-mL volumetric pipet (1); lower cabinet
25-mL volumetric pipet (2); lower cabinet
100-mL beaker (3); student drawer
250-mL beaker (2); student drawer
metal spatula (1); student drawer
stir bar (2); student drawer
magnetic stirrer/hotplate (2); table
ring stand (3); table
buret clamp (2); table drawer
utility clamp (1); table drawer
125-mL Erlenmeyer flask (4); upper cabinet
100-mL volumetric flask (2); upper cabinet
22
Reagents Used
•
•
•
•
NaOH standard solution
solid unknown weak acid
phenolphthalein or methyl red indicator solutions
pH 4, 7 and 10 buffer solutions
Preparing the Solution Containing the Unknown Weak Acid
Your group will prepare 100.00 mL of a solution containing the unknown weak acid.
•
Using an analytical balance, weigh about 0.7 g of the acid into a 100-mL beaker. Record
the exact mass of the acid to the nearest 0.0001 g.
•
Add approximately 25 mL of DI water to the beaker and swirl to dissolve the solid.
•
Carefully, transfer the solution to a 100.00-mL volumetric flask. Rinse the walls and
bottom of the beaker twice with DI water and transfer the rinses to the volumetric flask.
Cover the mouth of the flask with Parafilm and invert several times to dissolve the solid
acid. Add DI water to the mark on the flask, cover the mouth of the flask with Parafilm,
and invert several times to mix.
Titrating the Unknown Weak Acid Solution with NaOH
Each group has been allotted 150 mL of NaOH standard solution to perform the (2) colorimetric
and (1) potentiometric titrations of the unknown weak acid solution.
â–º Follow the general procedures described in the preceding sections on pp. 9-17.
Record the following:
1. Preparation of unknown acid solution, i.e. mass of acid, volume of water, etc.
2. Concentration of the NaOH standard solution
3. Volume of NaOH and unknown acid solutions used in the scout and careful
colorimetric titration
4. Volume of the unknown weak acid solution used in the potentiometric (pH)
titration
5. The screenshot of the potentiometric (pH) curve (pH vs. volume of NaOH added)
used to determine the experimental value(s) of Ka. Label the axes and provide a
title. Label the half-equivalence point(s) and the equivalence point(s) on the
graph to illustrate how you obtained the value(s) of Ka.
23
6. The screenshot of the potentiometric (pH) curve (the 2nd derivative) used to
determine the equivalence point. Identify and label the equivalence point.
Record the volume of titrant at the equivalence point.
WASTE DISPOSAL AND CLEANUP
•
All leftover solutions can be poured in the sink and rinsed down the drain with lots of
(tap) water. Do NOT discard the pH 4, pH 7 or pH 10 buffer solutions.
•
Rinse the stir bar and return it to your Graduate Instructor. BE CAREFUL! Do not pour
the magnetic stir bar into the sink with the solution.
•
Clean up any solids spilled on the balances or on the counter around them, dispose in sink
and rinse down the drain with plenty of (tap) water.
•
Return common equipment to your Graduate Instructor, or to the appropriate storage
location.
•
Disconnect the pH electrode and make sure the pH electrode is stored properly in DI
water or storage solution.
•
Keep your splash goggles on until you have completed the data analysis, obtained any
results that are needed from other groups, and are leaving the lab.
DATA ANALYSIS, RESULTS, AND DISCUSSION
Answer the questions and complete the calculations found on the Lab 9 Report
Form. The assignment is in the Labs module on Brightspace. Make sure you use
the version of the report form for on campus students.
When you have completed your report, upload it to Brightspace in the Lab 9
portal. It is due Mon. Apr. 5 by 11:59 PM ET.
24
10-
ê³µ
C
5-
0-
0
(11.58, 12.42)
10
30
40
20
volume (mL)
Second Derivative
10-
Second Derivative
0
0-
1

0
10
30
40
20
volume (mL)
ph
10-
5-
0-
0
10
30
40
20
volume (mL)
ph
0.5-
SecondDerivative
-0.5
-10
10
20
volume (mL)

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