CHARACTERIZATION OF WEAK ACIDS (#11.4)
The CCLI Initiative
Computers in Chemistry Laboratory Instruction
Learning Objectives
The objectives of this experiment are to ...
•
understand the titration curve for a weak acid.
•
calculate the molar mass of the weak acid.
•
calculate the Ka value for the weak acid.
•
use the molar mass and Ka values to identify the acid.
Background
In this experiment you will determine two important properties of an unknown weak
acid: its dissociation constant (Ka) and its molar mass. Both of these quantities can be
determined by titrating a known mass of the acid with a standard solution of sodium
hydroxide while electronically monitoring the pH. The unknown acid might be
monoprotic or diprotic. If the acid is monoprotic, the pH curve for the titration will Figure 1. pH versus
volume curve for the
resemble Figure 1. In this case the titration reaction is
titration of a monoprotic
HA + OHG ==> H 2 O + AG
and the Ka value for the acid can be determined by using the K a expression for the acid acid.
HA:
Ka = [H+][AG]
[HA]
If the weak acid being titrated is diprotic, a pH curve resembling that shown in Figure 2 will be
obtained. In this case the titration reaction to reach the first equivalence point is:
H 2 A
!
Y
!
+ OH
HA + H 2 O
Between the first and second equivalence points the titration reaction is
-
Y
HA + OHA 2G + H 2 O
The stoichiometry of the reactions, requires that for a pure acid, the volume to the second
Figure 2. pH
equivalence point be exactly twice that to the first equivalence point. to reach the first
versus volume
equivalence point. A diprotic acid has two dissociation constants, K al and K a2 The value of K al
curve for the
can be calculated from any point on the titration curve before the first equivalence point and
titration of a
diprotic acid.
that of K a2 between the first and second equivalence points. However, the best place to
determine either Ka is exactly half-way to either equivalence point, as that is where [HA] =
[AG], and therefor Ka = [H + ]
Experimental Procedure
•
KHP is dried for one hour at 110°C.
•
Boil one liter of de-ionized water and store in a capped Nalgene bottle.
•
Solutions are prepared and stored in capped
Nalgene bottle for use.
•
The sodium hydroxide solution is standardized.
•
Calibration of the drop counter to determine the
volume per drop for conversion of drops to volume.
•
Calibration of the pH electrode with buffers at 4, 7
and 10 pH.
•
Titration of an unknown acid.
•
Guidance is provided for the data analysis,
calculations and interpretation of titration curves.
•
A table of some 25 weak acids with their structures
is provided to assist in determining their unknown.
Figure 3. Classic titration curve for a weak acid with the 2nd
Resources Provided
derivative.
•
Sample Report Sheets providing the format to
organize the data collection with sample data.
•
Questions to consider, answer and turn-in with suggested answers.
•
Tips and Traps section to assist the instructor with potential problems and solutions.
•
Sample MicroLAB screen shots and graphs.
•
Laboratory preparation per student station.
www.microlabinfo.com
P.O. Box 7358
email: info@microlabinfo.com
Bozeman, MT
(888) 586 3274
59771-7358
The CCLl Initiative
LEARNING OBJECTIVES
The objectives of this experiment are to . . .
•
understand the titration curve for a weak acid.
•
calculate the molar mass of the weak acid.
•
calculate the K a value for the weak acid.
•
use the molar mass and K a values to identify the acid.
BACKGROUND
In this experiment you will determine two important properties of an unknown weak acid: its dissociation
constant (K a) and its molar mass. Both of these quantities can be determined by titrating a known mass of
the acid with a standard solution of sodium hydroxide while electronically monitoring the pH.
The unknown acid might be monoprotic or diprotic. If the acid is monoprotic, the pH curve for the
titration will resemble Figure 1.
In this case the titration reaction is
HA + OHG ==> H 2 O + AG
and the molar mass of the acid can be determined from
the volume of standard NaOH required to reach the
equivalence point:
moles of weak acid = moles of NaOH
Since the mass of weak acid titrated is known (by
weighing), the molar mass of the acid can be calculated.
1
Figure 1. pH versus volume curve for
the titration of a monoprotic acid.
The CCLl Initiative
The K a value for the acid can, in principle, be determined at any point in the titration by using the Ka
expression for the acid HA:
K a = [H+][AG]
[HA]
At a given point in the titration the [H +] can be calculated from the observed pH. The concentration of HA
can be calculated by subtracting the acid consumed in the titration to this point from the original moles of
acid and dividing by the volume:
original moles of HA - moles of HA consumed
total volume
The concentration of A ! present at a given point in the titration can be calculated from the moles of OH !
added to that point, since moles of OH - added equals moles of HA consumed which equals moles of A !
formed. Plugging these concentrations into the expression will give a value for K a.
If the weak acid being titrated is diprotic, a pH curve resembling that shown in Figure 2 will be obtained.
In this case the titration reaction to reach the first equivalence
point is:
H 2 A + OH
!
Y
HA
!
+ H 2O
Between the first and second equivalence points the titration
reaction is
HA
Figure 2. pH versus volume curve
for the titration of a diprotic acid.
-
+ OH-
Y
A 2G + H 2 O
As required by the stoichiometry of the reactions, the volume
required to reach the first equivalence point is identical to that
required to go from the first to the second equivalence point. This
volume along with the concentration of the NaOH can be used to
calculate the molar mass as in the case of a monoprotic acid.
A diprotic acid has two dissociation constants, K al and K a2. The
value of K al can be calculated from any point on the titration curve before the first equivalence point. The
value of K a2 can be calculated from points on the titration curve between the first and second equivalence
points. However, the best place to determine either K a is exactly half-way to the first equivalence point,
as that is where [HA] = [AG], and therefor K a = [H +]
The sodium hydroxide will be standardized by titrating a known mass of dry
potassium hydrogen phthalate (KHP), shown in Figure 3.
Figure 3. Structure of
KHP, potassium acid
phthalate.
2
The CCLl Initiative
SAFETY PRECAUTIONS
Safety goggles must be worn in the lab at all times. Any skin contacted with chemicals should be washed
immediately.
BEFORE PERFORMING THIS EXPERIMENT . . .
1.
Make sure you can locate the two Pre-designed programs you will need for this experiment,
(dropcal.exp and pH.vs.drop.titr.exp OR pH.vs.drop.Titr.0.1pH.exp) by clicking on the Titrations
tab.
2.
Dry 3.5 g KHP for one hour at 110 °C.
3.
Boil one liter of deionized water and store in a capped Nalgene bottle.
EXPERIMENTAL PROCEDURE
Solution preparation
A stock solution of NaOH is provided. Make 500 ml of 0.1 M NaOH using the stock solution and your
boiled deionized water. Store this solution in a capped Nalgene bottle for use as titrant The sodium
hydroxide solution will be standardized during the experiment.
Calibration of drop size
In this part of the experiment the volume of titrant will be determined by electronically counting
the number of drops required to reach the equivalence point. Therefore, the volume of a single drop must
be known accurately. It is important that the rate of delivery of the sodium hydroxide used in the
calibration part of the experim ent is the same as that used during the actual titration. This ensures
that the drop size remains constant.
1.
Your instructor will demonstrate how to set up the drop counter. Align the counter by filling the
buret with your NaOH solution and allowing it to drip through the counter into a small clean beaker.
The buret is correctly aligned when the counter light on the interface flashes every time a drop falls
from the buret. Alignment of the buret is not difficult, but is a critical step in producing an accurate
titration. Do not move the buret or the drop counter after they are aligned or serious errors may
result.
2.
The program dropcal.exp is provided in the Titrations tab to calibrate the drop counter. This will
give you the information necessary to calculate the volume of NaOH from the number of drops of
NaOH. If you are unfamiliar with how to do this, look in The Measurement Manual under
Calibrating Your Drop Counter.
3.
Read the buret and record the initial volume. Start the buret dripping at a moderately slow rate (two
to four seconds per drop). Count a minimum of 30 drops. Repeat your procedure five or six times.
Be sure to turn off your buret before exiting the program each time. Save each trial as a separate data
file.
3
The CCLl Initiative
Calibration of the pH electrode
1.
Attach the electrode to the interface unit at the pH position and place the electrode in a buffer
solution of pH 4.
2.
Click on pH in the Variables View and recalibrate your probe with the pH 4, 7 and 10 buffers
supplied. Be sure to rinse the pH probe with distilled water after each buffer and before you place it
in your analyte solution. Between titrations, the probe should be stored in the pH 7 buffer, then
rinsed well with distilled water before inserting into your titration beaker. If you are unfamiliar with
this, check the Calibrating your pH electrode in The Measurement Manual.
Standardization of NaOH
1.
Dissolve approximately 0.3 g of dry KHP in about 50 ml of boiled, deionized water. Be sure to use
the analytical balance, and record the exact mass of KHP used. Prepare two more samples in the
same manner.
2.
Refill the buret with 0.1 M NaOH. Check to be sure the alignment of the drop counter is
correct.
s t i l l
3.
Place a magnetic stirring bar in one of the KHP solutions. Put the pH electrode into the solution
taking care to position it out of the way of the stirring bar. Place the container under the buret and
counter assembly.
4.
The MicroLAB programs for the titration will be provided by your instructor under the file name
dropcal.exp and pH.vs.drop.titr.exp, OR pH.vs.drop.Titr.0.1pH.exp in the Titrations tab.
5.
Remember to titrate at about the same drop rate as in the calibration.
6. To maintain a constant head pressure keep the buret full by using the wash bottle to add
standardized NaOH.
7.
When the titration curve has leveled out, stop the experiment by clicking the Stop button.
8.
Run the program again for the other KHP sample. Be sure to name each data file appropriately so you
can identify it later.
Titration of an unknown solid mixture
1.
Dissolve approximately 1.0 g (record the exact mass using an analytical balance) of the unknown
solid mixture in about 70 ml of boiled, deionized water. Prepare three more samples in the same
manner
2.
Titrate the first two samples using phenolphthalein. Titrate your other two samples with your
standardized NaOH using the MicroLAB program indicated by your instructor.
3. Rinse the pH electrode with deionized water when you are finished
4
The CCLl Initiative
DATA ANALYSIS
1.
From your calibration data, calculate the milliliters of titrant per drop.
2.
Click on the Add Formula button and input the formula to convert drops to volume, using the ml of
titrant per drop ratio you calculated above. See the Appendix if you don’t know how to do this.
3.
Click and drag this formula from the Data Sources/Variables view to the Graph Y2 Axis and a
Spreadsheet column. Remember: The independent variable goes on the X axis.
4.
After you have obtained the approval of your instructor, print the graph and record the
calibration equation for use in your titrations.
Standardization of NaOH
1.
For each of the MicroLAB titrations, calculate the equivalence point volume by determining the first
and second derivative according to the instructions in the appendix and plot them on each graph. You
will have to make two graphs per titration, one for the titration curve plus the first derivative, and the
second for the titration curve plus the second derivative.
2.
For the titrations using phenolphthalein, calculate the equivalence point volume from the normal
stoichiometric calculations.
3.
Compare the equivalence point volumes for the two methods, and answer the following questions:
a. Calculate the percent difference between the two sets of data.
b. Which data set do you think is the most accurate? Why?
c. What would the percent error be in the amount of KHP if you used the phenolphthalein set of
data?
4.
Determine the concentration of your NaOH solution from the equivalence point and the
KHP from the average of the two sets of data.
m ass of
Determination of the Ka and molar mass of an unknown acid
1.
Open your data from the titrations of your unknown acid into the MicroLAB.
2.
Using the Analysis functions, construct the first and second derivatives for your unknown acid for
each of the four titrations. See The M easurement Manual for detailed instructions on how to do
this.
3.
Convert the drops for each into volume as you did for the standardization, determine the equivalence
point(s) for each titration of your unknown acid.
4.
Average this data and calculate the molar mass of your unknown acid.
5.
Calculate values for the dissociation constant for the acid. If you have a diprotic acid, calculate both
K al and K a2.
6.
From the list of possible unknown acids in Table 1, determine the identity of your unknown acid.
5
The CCLl Initiative
6
The CCLl Initiative
7
Learning Objectives
The objectives of the experiment are to . . .
•
understand the titration curve for a weak acid.
•
calculate the molar mass of the weak acid.
•
calculate the dissociation constant for the weak acid.
•
use the molar mass and values to identify the acid.
Procedure Overview
•
after calibration of drop size and the pH electrode, a solution of NaOH is standardized with KHP.
•
an unknown weak acid is titrated with the standardized base. The molar mass and dissociation constant
for the acid are calculated.
•
the identity of the unknown acid is determined from the molar mass and Ka values.
9
Name
Section __________ Date __________________
CHARACTERIZATION OF A WEAK ACID
Report Sheet
Calibration of drop size
Give the equation for conversion from drops NaOH to ml NaOH:
Standardization of NaOH
Trial 1
Trial 2
data file name
__________
__________
mass KHP
__________ g
__________ g
molarity NaOH
__________ M
__________ M
Average molarity
__________ M
Titration of unknown acid
Unknown # _____
Trial 1
Trial 1
Trial 1
Trial 1
data file name
__________
__________
__________
__________
mass of acid
________ g
________ g
________ g
________ g
volume H2O
________ml ________ml ________ml ________ml
molar mass of acid
__________
__________
__________
__________
Average molar mass __________
Kal values
__________
__________
__________
__________
Ka2 values
__________
__________
__________
__________
Average
Ka1_______ Ka2_______
Identity of unknown acid ______________________________________
10
Name
Section __________ Date __________________
CHARACTERIZATION OF A WEAK ACID
Report Sheet (page 2)
Calculations
Standardization of NaOH
1. Determine the moles of KHP.
2. Calculate the molarity of NaOH for each trial.
3. Calculate the average molarity of NaOH.
Titration of unknown acid
1. Calculate the molar mass for the acid.
2. Determine the dissociation constant(s) for the acid at a minimum of three points along the curve.
11
Name
Section __________ Date __________________
CHARACTERIZATION OF A WEAK ACID
Questions/Problems
1. A 33.50 ml volume of 0.1050 M NaOH was required to just neutralize a 0.2500 g sample of a
monoprotic organic acid dissolved in 50.00 ml of water. What is the molar mass of the acid?
If the dissociation constant of the acid were 3.0 x 10-6, what would be the pH
2. Indicate how each of the following would affect the values of and molar mass for a weak acid
calculated in this experiment. (In other words, would the Ka and molar mass obtained be higher,
lower, or unchanged from what it should be?)
a. the molarity of the NaOH is actually greater than believed.
b. the pH meter reads 0.60 high all the time.
c. the sample of weak acid was actually impure, containing about 5% of an inert material.
12
Name
Section __________ Date __________________
CHARACTERIZATION OF A WEAK ACID
Questions/Problems (page 2)
3. A 0.0050 mole sample of a weak acid with a dissociation constant of 1.0 x 10-7 was titrated with
0.200 M NaOH, after being dissolved in 50.0 ml of water. Calculate the pH of the solution.
a. before any NaOH was added.
b. after 10.0 ml of NaOH was added.
c. after 24.9 ml ofNaOH was added.
d. after 25.1 ml ofNaOH was added
13
CHARACTERIZATION OF A WEAK ACID
Tips and Traps
1. Students must use boiled, deionized water for their solutions.
2. It is helpful to show students the proper titration set-up. The tip of the buret should be level with
the top of the drop counter and centered on the cross lines on the case for optimum results.
3. Students should take time to align the drop counter properly. Most problems in the titrations
result from a poorly aligned counter. No program is necessary for alignment. All students need
to do is start the buret dripping and watch the counter light on the interface. If it blinks at each
drop, alignment is correct.
4. The KHP should be dried at 110°C for two hours.
5. Unknown acids should NOT be dried. Some of them will decompose.
6. It is easiest to take all data for all trials at one sitting instead of making one solution at a time.
Using the MicroLAB tools
Instructions on the use of the MicroLAB tools such as probe calibration, use of the drop counter,
obtaining derivatives and interpolating between titration points are given in Useful Titration
Operations within the Useful Tools folder. Their first derivative graphs may look similar to the
following graph.
There is a lot of
noise around the
equivalence point
which results from
a
s m a l l
irregularities in the
pH caused by
mixing of the
solution and the lag Figure 2. HOAc titration curve by drop
Figure 1. HOAc titration curve by drop
time in the probe to counter, with data taken every 0.1 pH units
counter, with first derivative overlay.
measure pH. When and with first derivative overlay resulting
in classic derivative plots.
the data is read
every 0.1 pH units, it produces a classic first and second
derivative as seen in Figure 2.That peak point is closest to the equivalence point. The second
derivative at that point, going from positive to negative, will then give the exact equivalence point.
This then gives the volume of titrant required to reach the equivalence point, from which can be
calculated the number of moles of the analyte.
14
CHARACTERIZATION OF A WEAK ACID
Tips and Traps
Titration Program: The following program will result in the type of titration curve and
derivative as seen in the figure.
Acid/Base Titration, data taken every 0.1 pH units. Use for any titration of an acid with a base, or a
base with an acid, recording the data each time the pH has changed by 0.1 pH units. Use of this program
results in very smooth titration curves and generally very well shaped derivatives.
Experiment name: . pH.tem p.vs.drop.titr.0.1pH.exp.
Sensors: drop counter: X axis, Col. A, DD on top, units = drops; pH: Y1 axis, Col B, DD in middle,
units = pH; Temp: Y2 axis, Col C, DD on bottom, units °C. (Use of temperature at instructors
discretion.)
Special Program:
Read Sensors
Repeat when counter change (Sets to read only when a drop has passed through the counter.)
If Delta pH > +/- 0.100 (Sets to read only when pH has changed by 0.1 pH units.)
Read Sensors (Reads all variables selected in Data Sensors/Variables and stores in a data grid.)
Else
End If
Until Stop Button is pressed
Comment: Calibrate the drop counter using one of the dropcal.exp before and after the series of titrations
. If temperature is measured with a Temp(IC) probe, it must be wrapped in Saran Wrap to prevent
grounding the pH probe. This program can be saved as a Template. See Measurement Manual
for details.
Titration curve of HOAc with pH versus
drop counter values. In this titration the
pH was taken only after the pH had
changed by 0.1 pH units. Note that the
curve is much smoother, and the first
derivative is classic in shape.
Titration curve of HOAc with pH verses drop counter
values, taken at o.1 pH values, with second derivative
classic shape.
15
CHARACTERIZATION OF A WEAK ACID
Suggested Answers to Questions/Problems
1. A 33.50 ml volume of 0.1 050 M NaOH was required to just neutralize a 0.2500 g sample of
a monoprotic organic acid dissolved in 50.00 ml of water. What is the molar mass of the
acid? If the dissociation constant of the acid were 3.0 x 10-6, what would be the pH at the
equivalence point?
Molar Mass = 71.07 g
pH=3.45
2. Indicate how each of the following would affect the values of and molar mass for a weak
acid calculated in this experiment. (In other words, would the Ka and molar mass obtained be
higher, lower, or unchanged from what it should be?)
a. the molarity of the NaOH is actually greater than believed.
Ka: lower
MM: higher
b. the pH meter reads 0.60 high all the time.
Ka: higher
MM: unchanged
c. the sample of weak acid was actually impure, containing about 5% of an inert material.
Ka: lower
MM: higher
3. A 0.0050 mole sample of a weak acid with a dissociation constant of 1.0 x 10-7 was titrated
with 0.200 M NaOH, after being dissolved in 50.0 ml of water. Calculate the pH of the
solution.
a. Before any NaOH was added.
pH = 4.00
b. After 10.00 ml of NaOH was added
pH = 6.82
c. After 24.9 ml of NaOH was added
pH = 9.40
d. After 25.1 ml of NaOH was added
pH= 10.42
16
CHARACTERIZATION OF A WEAK ACID
Sample Data
Calibration of drop size
Students should provide the equation for conversion from drops NaOH to ml NaOH:
Standardization of NaOH
Trial 1
0.5910 g
__________
0.09516 M
Average molarity
Trial 2
0.6020 g
__________
0.09482 M
0.049499 M
Trial 1
Trial 1
Trial 1
Trial 1
mass of acid
0.3937 g
0.3152 g
0.3843 g
0.3521 g
volume H2O
________ml ________ml ________ml ________ml
data file name
__________
__________
__________
__________
molar mass of acid
150.9
152.1
152.7
151.9
mass KHP
data file name
molarity NaOH
Titration of unknown acid
Unknown # _____
Average molar mass 151.9
(Actual 152.15)
Kal values
3.9 x 10-4
3.5 x 10-4
3.5 x 10-4
Ka2 values
__________
__________
__________
Average
Ka13.6 x 10-4 Ka2_______ (actual 1.4 x 10-4)
__________
Identity of unknown acid ___Mandelic acid___________________________________
18
CHARACTERIZATION OF A WEAK ACID
Calculations
Calculations
Standardization of NaOH
1. Determine the moles of KHP.
0.5910 g KHP
204.3 g/mol
= 2.893 xl0-3 mol KHP
2. Calculate the molarity of NaOH for each trial.
2.893 X 10 -3 mol OH - = 0.09516 M
30.41 x 10-2 L
3. Calculate the average molarity of NaOH.
0.09516 M + 0.09491 M
3
+
0.09474 M
= 0.9499 M
Titration of unknown acid
1. Calculate the molar mass for the acid.
475 x 0.05784 ml x 0.09499 mmol = 2.6098 mmol
drop
ml
0.3937 g
2.6098 x 10-3 mol
= 150.9 g/mol
2. Determine the dissociation constant(s) for the acid at a minimum of three points along the
curve.
Varies with unknown . . .
19
CHARACTERIZATION OF A WEAK ACID
Laboratory Preparation (per student station)
Equipment
•
pH electrode
•
ring stand
•
MicroLAB drop counter
•
buret with buret clamp
•
Nalgene bottle
•
wash bottle (for NaOH)
•
250 ml beakers
Supplies
•
paper towels
Chemicals
•
KHP
•
unknown acids (2.5 - 2.6 g samples), see attached list
•
sodium hydroxide stock solution (6 M)
•
buffer solution (pH 7.0)
Safety and Disposal
•
no special precautions needed
Unknown Acids (Use 2.5 - 2.6 gram samples)
•
adipic acid
•
benzoic acid
•
o-bromobenzoic acid
•
o-chlorobenzoic acid
•
p-chloropropanoic acid
•
dimethylmalic acid
•
p-hydroxybenzoic acid
•
mandelic acid
•
malonic acid
•
o-nitrobenzoic acid
•
oxalic acid
•
phenylacetic acid
•
potassium hydrogen phosphate
•
salicylic acid
•
sulfanilic acid
•
tartaric acid
a-bromoacetic acid
a-chloroacetic acid
3-chloropropanoic acid
fumaric acid
lactic acid
maleic acid
phthalic acid
potassium hydrogen phthalate
succinic acid
trimethvlacetic acid
20