Purpose:
The purpose of this lab is to identify an unknown diprotic acid (RED
tape sample) by finding its molecular weight through titration and to observe
the dissociation and protonation of a diprotic acid by graphing the titration.
Background:
An acid/base titration is performed by carefully adding one solution
from a buret, the titrant, to another substance in a beaker until all of the
substance in the flask has reacted.
A diprotic acid is an acid that dissociates in two stages and yields two
+
H ions per acid molecule:
(1) H2X(aq) 

 H+(aq) + HX-(aq)
(2) HX-(aq) 

 H+(aq) + X2-(aq)
Because of the successive dissociations, the titration curves of diprotic acids
have two equivalence points. The equations for the acid-base reactions
occurring between a diprotic acid, H2X, and NaOH, are:
from the beginning to the 1st equivalence point:
(3) H2X + NaOH 

 NaHX + H2O
from the 1st to the 2nd equivalence point:
(4) NaHX + NaOH 

Na2X + H2O
from the beginning of the reaction through the 2nd equivalence point (net
reaction):
(5) H2X + 2 NaOH 

Na2X + 2 H2O
The titration curve shown below is a general graph of the dissociation
of a diprotic acid. In the process, there are two buffering regions and two
equivalence points.
Titration Curve of a Strong Base Titrating a Polyprotic Acid
In this experiment, an unknown diprotic acid is titrated with a NaOH
solution of known concentration. Weighing the given sample of acid will
reveal its mass in grams, and the number of moles of the acid can be
determined from the volume of NaOH titrant needed to reach the 1st
equivalence point, because by definition the equivalence point is when
chemical equilibrium, in which the moles of the acid and the moles of the base
are the same, is reached. The volume and the concentration of NaOH titrant are
used to calculate moles of NaOH. Once grams and moles of the diprotic acid
are known, molecular weight can be calculated, in g/mole. Molecular weight
determination is a common way of identifying an unknown substance in
chemistry.
Either the 1st or 2nd equivalence point can be used to calculate the
molecular weight of the diprotic acid. If the second equivalence point is more
clearly defined on the titration curve, simply divide its NaOH volume by 2 to
confirm the first equivalence point; or from Equation 5, use the ratio: 1 mole
H2X / 2 mol NaOH.
Materials:
 Lab safety goggles
 TI Graphing Calculator
with DataMate program
 CBL 2 interface
 Vernier pH Sensor
 Unknown diprotic acid,
.120g
 Milligram balance
 50mL buret
 0.1M NaOH solution







Ring stand
(2) utility clamps
50mL buret
250ml beaker
Graduated cylinder
Wash bottle filled with
distilled water
Magnetic stirrer & stirring
bar
Procedure:
1) Obtain and wear safety goggles.
2) Weigh out about 0.120g of the unknown diprotic acid on a piece of paper.
Record the mass to the nearest 0.001g in the data table.
3) Transfer the unknown acid to a 250mL beaker and dissolve it in 100mL of
distilled water.
4) Place the beaker onto a magnetic stirrer & add a small stirring bar.
5) Calibrate the pH sensor, and then plug it into channel 1 of the CBL 2
interface. Use the link cable to connect the TI Graphing Calculator to the
interface.
6) Use a utility clamp to suspend a pH Sensor on a ring stand. Position the pH
Sensor in the in the diprotic acid solution and adjust its position toward the
outside of the beaker so that it will not be struck by the stirring bar.
7) Obtain a 50mL buret, rinse it with a few mL of 0.1 M NaOH solution, and
fill it with NaOH just a little over the 0mL mark. Then drain a small amount of
NaOH solution so it fills the buret tip and leaves the NaOH at the 0-mL level
of the buret.
7) Turn on the calculator and start the DATAMATE program. Press CLEAR
to reset the program.
8) Set up the calculator and interface for the pH Sensor.
a. Select SETUP from the main screen
b. If CH 1 displays PH, proceed directly to step 9. If it does not,
continue with this step to set up your sensor manually.
c. Press ENTER to select CH 1.
d. Select PH from the SELECT SENSOR menu.
9) Set up the data-collection mode.
a. To select MODE, press “^” once and press ENTER.
b. Select EVENTS WITH ENTRY from the SELECT MODE menu.
c. Select OK to return to the main screen.
10) Select START to begin data collection. Before adding any NaOH solution,
press ENTER and type in “0” as the buret volume in mL. Press ENTER to save
the 1st data pair for the experiment.
11) Add the next increment of NaOH, enough to raise the pH about 0.20 units.
When the pH stabilizes, press ENTER and enter the current buret reading (to
the nearest 0.01mL).
12) Continue adding NaOH solution in increments that raise the pH by about
0.20 units and enter the buret reading after each increment. Proceed in this
manner until the pH is 3.5.
13) When pH 3.5 is reached, change to 2-drop increments. Enter the buret
reading after each increment.
14) After pH 4.5 is reached, again add larger increments that raise the pH by
about 0.20 units and enter the buret reading after each addition. Continue in
this manner until a pH of 7.5 is reached.
15) When pH 10 is reached, again add larger increments that raise the pH by
0.20 units. Enter the buret reading after each increment. Continue in this
manner until you reach a pH of 11 or use 25mL of NaOH, whichever comes
first.
16) Press STOP when you have finished collecting data. Then examine the
data on the displayed graph to find the equivalence point – that is the largest
increase in pH upon the addition of 1 drop of NaOH solution. As you move the
cursor right or left on the displayed graph, the volume (x) and pH (y) values of
each data point are displayed below the graph. One of the two equivalence
points is usually more clearly defined than the other; the two-drop increments
near the equivalence points frequently result in larger increases in pH at one
equivalence point than the other. Indicate the more clearly defined equivalence
point (1st or 2nd) in the data table.
17) Determine the volume ofo NaOH used for the equivalence point you
selected. To do so, examine the data to find the largest increase in pH values
during the 2-drop additions of NaOH. Find the NaOH volume just before this
jump. Then find the NaOH volume after the largest pH jump. Record these
values in the data table. For the alternate equivalence point (the one you did
not use in the previous step,) examine the data points on your graph to find the
largest increase in pH values during the 2-drop additions of NaOH. Find the
NaOH volume just before and after this jump. Record these values in the data
table.
18) Dispose of the beaker contents down the drain.
19) Rinse the pH sensor and return it to the pH storage solution.
20) Before printing the graph of pH vs. volume, rescale the y-axis from 0-12
pH units, with increments of 1 pH unit.
21) Print a copy of the graph of pH vs. volume, and using graphical analysis
software, print a copy of the NaOH volume and pH data for the titration.
Safety Precautions:
Handle the solid acid and its solution with care. Acids can harm your
eyes, skin, and respiratory tract.
Sodium hydroxide solution is caustic. Avoid spilling it on your skin or
clothing.
Data:
Mass of diprotic acid
.12g
Concentration of NaOH
.01 M
Equivalence pt.
1st equivalence pt.
pH = 4.13
2nd equivalence pt.
pH = 9.07
NaOH volume added before and after
1st volume
3 mL
5 mL
the largest pH increase
2nd volume
7 mL
Volume of NaOH added at the
1st volume
4 mL
equivalence point
2nd volume
8 mL
Moles of diprotic acid
.001 mols
Moles of NaOH
1st equivalence pt.
.04 mols
2nd equivalence pt.
.08 mols
9 mL
Name, formula, and accepted
Malic acid
molecular weight of the diprotic acid
H2C4H4O5
116 g
Volume at equilibrium pt.
1st volume
8.99 mL
2nd volume
4.69 mL
Observations:
Initially, as drops of NaOH base reacted with the diprotic acid in the
beaker, they turned pink, but the color quickly dissipated. As more mols of
base were added to the acid, the solution completely turned into a light pink
and stayed pink but continued to change into an even darker pink as more base
was added.
Analysis:
At the first equivalence point, all H+ ions from the first dissociation
have reacted with NaOH base. At the second equivalence point, all H+ ions
from both reactions have reacted (twice as many as at the first equivalence
point). Therefore, the volume of NaOH added at the second equivalence point
is exactly twice that of the first equivalence point. To find the volume of
NaOH, take an average of the number just before the equivalence point is
reached and at the peak of the equivalence point. To calculate of moles of
NaOH one must multiply the concentration of NaOH by the volume of NaOH
added at the equivalence point. The moles of diprotic acid can be found by
dividing the moles of NaOH by 2. This is because the diprotic acid has two
H+ ions and the base only has one Na+ ions. To find the molecular weight of
the diprotic acid, divide the grams of unknown acid into the moles of your
diprotic acid.
Our percent error was calculated to be zero percent. This lab produced
a gram molecular weight of malic acid very close to the actual accepted
molecular weight; therefore, lab error was relatively little.
Purpose:
The purpose of this lab is to identify an unknown diprotic acid (RED
tape sample) by finding its molecular weight through titration and to observe
the dissociation and protonation of a diprotic acid by graphing the titration.
Conclusion:
The unknown diprotic acid (RED tape sample) used in the titration was
malic acid, H2C4H4O5. The experimentally determined molecular weight of the
diprotic acid was 134g/mol, which has a 0.00% error from the accepted
molecular weight of 134g/mol.
When polyprotic acids are titrated with strong bases, there are multiple
equivalence points because polyprotic acids lose their protons in a stepwise
manner.At either equivalence point, there were twice as many mols of NaOH
base for every mol of diprotic acid because there is 2:1 stoichiometric ratio
between the two species of the reaction. This fundamental mathematical
relationship between NaOH and the diprotic acid used in the titration allowed
for the subsequent calculation of the grams of the unknown diprotic acid and
revealed its identity.