EXPERIMENT
2
What Chemical Species Control the
Conductivity of Aqueous Solutions?
Original Experiments from Chemistry with Vernier. Adapted by Chemistry, Brown University
INTRODUCTION
Electrolytes
Two major categories of chemical compounds are ionic and molecular substances. Ionic substances consist of
ions of opposite charge that are held together by electrostatic forces. The structure of an ionic compound is a
large crystal array that maximizes the attractive forces of the oppositely charged ions. In contrast, the atoms
in molecular substances are not ions and they are held together by covalent bonds (shared electrons). The
structure of a molecular substance is determined by the bonding arrangement of the atoms. One illustrative
way to distinguish between the two chemical compounds is by dissolving the compounds in water and
monitoring the resulting aqueous solution for electrical conductivity. Ions that are released in aqueous
solution will conduct electricity while molecules released in solution will not. Based on their ability to
conduct an electrical current, ionic and molecular substances are classified as strong electrolytes, weak
electrolytes or non-electrolytes. The examples below show the hydration reactions for various substances that
demonstrate the three major classes of electrolytes.
Ionic Compounds
These are usually strong electrolytes and can be expected to dissociate 100% in aqueous solution.
NaNO3(s) + H2O(l) à Na+(aq) + NO3- (aq)
(Eq. 1)
Molecular Compounds
These are usually non-electrolytes. They do not dissociate to form ions. Resulting solutions do not conduct
electricity.
CH3COCH3(l) + H2O(l) à CH3COCH3(aq)
(Eq. 2)
Precipitation Reactions and Conductivity
NOTE: Molecular substances can dissolve in aqueous solution by forming intermolecular forces with the
solvent.
Molecular Acids
These are molecules that can partially or wholly dissociate, depending on their strength.
+
HNO3(aq) + H2O (l) à H (aq) + NO3-(aq) (100% dissociation)
O(l)àH+ (aq)+
HF(aq)+H2
-
F (aq) (<<<100% dissociation)
(Eq. 3)
(Eq. 4)
In part A of this experiment, observations of various substances in aqueous solutions will be monitored using
a Conductivity Probe. When the probe is placed in a solution that contains ions, and thus has the ability to
conduct electricity, an electrical circuit is completed across the electrodes that are located on either side of the
hole near the bottom of the probe body (see Figure 1). This results in a conductivity value that can be read by
LabQuest. The unit of conductivity used in this experiment is the microsiemens per centimeter, or µS/cm .
Aqueous solutions of various chemical substances will be classified as strong, weak or non-electrolytes based
on their conductivity. Three unknown solutions will then be analyzed for conductivity and their identity will
be determined by comparison of conductivity values of the known solutions.
When two aqueous solutions of ionic compounds are mixed together, a double displacement reaction occurs
and a precipitate may form as shown below. The formation of a precipitate is controlled by the concentration
1 and solubility of the various ions in solution. Prior to mixing the two aqueous solutions, both solutions would
demonstrate conductivity. Once the solutions are mixed, the resulting solution and precipitate mix would still
demonstrate conductivity based on equation 6. For precipitation reactions such as shown in equation 5,
measuring the conductivity of the reaction would not be a useful way to monitor the equivalence point of the
reaction. Recall that the equivalence point of a reaction is when the moles of both reactants are equal. At the
equivalence point for the reaction in equation 5, the counter ions (Na+ and NO3-­‐) will still be present in
solution as shown more clearly in equation 6. There will be no distinct change in conductivity that would
signal the equivalence point in this type of reaction.
Molecular equation
AgNO3(aq) + NaCl(aq) àAgCl(s) + NaNO3(aq)
(Eq. 5)
Complete ionic equation
+
Ag (aq) + NO (aq) + Na+ (aq) + Cl-­‐(aq) à AgCl(s) + Na+(aq) + NO (aq)
(Eq. 6)
Net ionic equation
Ag+(aq) + Cl-­‐(aq) à AgCl(s)
(Eq. 7)
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However, a double displacement reaction that yielded a precipitate and a non-electrolyte product would not
demonstrate conductivity at the equivalence point. The absence of ions at the equivalence would clearly
identify this point if the conductivity is monitored throughout the reaction.
The reaction between aqueous barium hydroxide and aqueous sulfuric acid is such a double displacement
reaction as shown in Equation 8.
Molecular equation
Ba(OH)2(aq) + H2SO4(aq) à BaSO4(s) + 2 H2O(l)
(Eq. 8)
In Part B of this experiment, the conductivity during the reaction between sulfuric acid, H2SO4 and barium
hydroxide, Ba(OH)2, is monitored to determine the equivalence point. Changes in the conductivity of the
solution will be monitored using a Conductivity Probe attached to the LabQuest. From this information, the
concentration of the Ba(OH)2 solution will be determined. The effect of ions, precipitates, and water on
conductivity will be observed.
PROCEDURE (WORK IN PAIRS)
Safety Precautions
Acids and bases are corrosive and should be handled with care. Ba(OH)2 is toxic (solution is very
dilute) and care should be taken not to spill the solution. If any reagents contact the skin, wash off
immediately.
Materials
Part A
General Equipment
250 mL beaker
Distilled water wash bottle
Special Equipment
Reagents
Conductivity probe with LabQuest
2 0.050 M NaCl
0.050 M CaCl2
0.050 M AlCl3
0.050 M CH3CO2H
0.050 M H3PO4
0.050 M H3BO3
0.050 M HCl
0.050 M CH3OH
0.050 M HOCH2CH2OH
0.050 M of the following labeled as
unknowns:
CH3CH2OH
MgCl2
CH3CH2CO2H
Part B
General Equipment
Special Equipment
2 x50 mL Beaker
Conductivity probe with LabQuest
Stir plate and stir bar
50 mL buret
128mL graduated
Chemistry 0330
Laboratory Manual
Brown University
100
cylinder
Buretnclamp
Reagents
0.0200 M H2SO4
Unknown concentration of Ba(OH)2
Part A: Properties of
1. Assemble the Conductivity Probe, utility clamp, and ring stand as
shown in Figure 1. Be sure the probe is clean and dry before beginning
the experiment.
1. Assemble the Conductivity Probe,
utility clamp, and ring stand as shown in Figure 1. Be sure the probe
Part
A: Properties of Solutions
Solutions
! Vernier Software and Technology. Used with permission.
is clean and dry before beginning the experiment.
Figure 1
The Conductivity Probe,
utility clamp and ring stand
Figure 1
The Conductivity Probe,
utility clamp and ring stand
2. On the LabQuest, choose New from the File menu. Set the selector switch on the side of the
2. Set the selector switch on the side of the Conductivity Probe to the
Conductivity Probe to the 0–20000 µS/cm range initially. If the conductivity value is < 200 µS/cm, or if
0–20000 mS/cm range. On the LabQuest, choose New from the File
your meter reads zero, you can switch to a more sensitive scale: 0-200 µS/cm. You should not have a zero
reading for any of the solutions.menu.
3. Obtain the Group A solutions in the provided vials. The solutions are:
0.050 M CaCl2, 0.050 M NaCl, and 0.050 M AlCl3.
3. Obtain the Group A solutions in the provided vials. The solutions are: 0.050 M CaCl2, 0.050 M NaCl,
4. Measure the conductivity of each of the solutions.
and 0.050 M AlCl3.
a. Carefully raise each vial and its contents up around the Conductivity Probe until the hole near the probe end is completely suba. Carefully raise each vial andmerged
its contents
around
the Conductivity
Probe until
the holeSince
near thethe two
in up
the
solution
being tested.
Important:
probe end is completely sub- merged
in
the
solution
being
tested.
Important:
Since
the
two
electrodes
electrodes are positioned on either side of the hole, this part of the
are positioned on either side of the hole, this part of the probe must be completely submerged.
probe must be completely submerged.
4. Measure the conductivity of each of the solutions.
b. Briefly swirl the vial contents. Monitor the conductivity reading
displayed on the screen for 6–8 seconds, then record the value in
your notebook.
3 c. Before testing the next solution, clean the probe by surrounding
the probe with a 250 mL beaker and rinse it with distilled water
b. Briefly swirl the vial contents. Monitor the conductivity reading displayed on the screen for 6–8
seconds, then record the value in your notebook.
c. Before testing the next solution, clean the probe by surrounding the probe with a 250 mL beaker and
rinse it with distilled water from a wash bottle. Blot the outside of the probe end dry using a KimWipe.
It is not necessary to dry the inside of the hole near the probe end. Be sure to dispose of the rinses in
the waste container at the end of the experiment.
d. The solutions in the vials will be reused. When you are finished with the Group of solutions, make
sure the caps are tight and then return the vials to the bench in the back of the lab.
5. Obtain the four Group B solutions that include 0.050 M CH3CO2H, 0.050 M HCl, 0.050 M H3PO4, and
0.050 M H3BO3. Repeat the Step 4 procedure.
6. Obtain the two Group C solutions that include 0.050 M CH3OH, and 0.050 M HOCH2CH2OH. Also
place distilled water and tap water in separate labeled beakers that will be included in Group C. (Don’t
forget about these). Repeat the Step 4 procedure.
7. Obtain the Unknown Group solutions and record the letter codes in your notebook. Repeat the Step 4
procedure.
Part B: Conductivity and the Equivalence Point
Figure 2
Part B equipment setup
Stir bar
1. Measure out approximately 60 mL of 0.02000 M H2SO4 into a clean and dry 150 mL beaker. Record the
precise H2SO4 concentration in your notebook.
2. Obtain the 50 mL buret and wash it with soap and tap water in the sink area. Practice how to use burets
with tap water. Make sure that the buret doesn’t leak water or is broken. Rinse it with a few mL distilled
water. Take the buret to the hood. Hold the buret with a buret clamp and use a small, clean funnel to rinse
the buret with a few mL of the H2SO4 solution (drain the solution into a waste beaker). Fill the buret a little
above the 0.00 mL level and drain a small amount of H2SO4 solution so it fills the buret tip and expels any
air bubbles. (ask TA for help if you have trouble expelling air bubbles). Adjust the volume of H2SO4 to
the 0.00 mL level of the buret.
4 3. Using the provided pump dispenser in the hood, dispense 25.0 mL of Ba(OH)2 of unknown concentration
into a clean and dry 100 mL graduated cylinder and record the exact volume. Then pour the solution into a
clean 150 mL beaker. Record the letter code of the Ba(OH)2 solution. Then add 60 mL of distilled water to
the beaker and a stir bar.
4. Set the selector switch on the side of the Conductivity Probe to the 0–2000 µS/cm range. On the
LabQuest, choose New from the File menu.
5. Set up the data-collection mode.
a. On the Meter screen, tap Mode. Change the data-collection mode to Events with Entry.
b. Enter the Name (Volume) and Units (mL). Select OK.
6. Arrange the buret, the Conductivity Probe, the beaker containing Ba(OH)2, on the stir plate as shown in
Figure 2. The Conductivity Probe should extend down into the Ba(OH)2 solution to just above the stirring
bar, so the hole in the probe end is completely submerged.
7.
a. Start data collection by touching the green button on the lower left side of the screen.
b. Before adding H2SO4 titrant, tap Keep (next to the start/stop button with a star after the word Keep).
Enter 0.00, the volume of the buret. All buret readings should have 2 decimal places. Select OK to
save this data pair. Please keep in mind that you are recording the volume delivered from the buret. By
starting with the solution at 0.00, the amount delivered and the buret reading will be the same.
c. Add 1.00 mL of 0.0200 M H2SO4 to the beaker. When the conductivity readings stabilize, tap Keep.
Enter the exact reading of the buret (it is fine if slightly more or less of 1 mL was added) and then
select OK. The conductivity and volume values have now been saved for the second reading.
d. When the conductivity has dropped below 500 µS/cm , add H2SO4 solution in 0.5 mL increments
(or less), entering the exact buret reading each time.
e. After the conductivity has dropped below 200 µS/cm, add H2SO4 in 2-drop increments (≈0.1 mL)
and enter the buret reading.
f. Continue adding H2SO4 solution in 2-drop increments (≈ 0.1 mL) until the minimum conductivity
has been reached at the equivalence point. Enter the volume after each 2-drop addition. When you
have passed the equivalence point, continue using 2-drop increments until the conductivity is greater
than 50 µS/cm again.
g. Now use 1.00 mL increments until the conductivity reaches about 1000 µS/cm , or about 13 mL of
H2SO4 solution has been added, whichever comes first.
8. Stop data collection by touching the red button on the lower left side of the screen. To view the data as a
graph, touch the Graph icon on the top right hand side of the screen. To view the data in a table format,
touch the Table icon on the top right hand side. While in the graph or table mode, insert your USB flash
drive into the LabQuest. Choose Save from the File menu and give the file a name. To save the data on the
USB flash drive, touch the USB flash drive icon. Do not save data on the LabQuest!
9. To examine the data pairs on the displayed graph of conductivity vs. volume (must be in the Graph
mode), tap any data point. As you tap each data point (or use the > or < keys on LabQuest), the
conductivity and volume values are displayed. Find the equivalence point—that is, the volume when the
conductivity value reaches a minimum.
10. Run a second trial of the titration using a fresh sample of Ba(OH)2 and following steps 3–8. Be sure to
clean the Conductivity Probe with distilled water for the new trial. DO NOT bring the buret volume to 0.00
5 mL for the second trial. Simply subtract the initial buret reading from the final buret reading in your
notebook and enter the difference in the LabQuest. Record all the data in your lab notebook.
11. When finished with the second trial, rinse the Conductivity Probe with distilled water. Check to make
sure that your experiments were not saved to the LabQuest. Without the USB flash drive in place, go to File
and touch Open. If anything is listed, you need to delete the files. To delete files, go to File and choose
Delete. Highlight the files to be deleted by touching with the stylist pen. Hit Delete and then touch Done to
return to the main screen. Turn off the LabQuest by pressing on the silver button on the top left. Leave the
LabQuest and Probe in the workstation. Rinse the buret with a few mL of distilled water and collect the
rinse as waste. Use more water for the second rinse of the buret but do not collect this in the waste
container. Return the clean buret and buret clamp to the bench in the back of the lab.
Place all used and unused solutions in the waste container in the hood at the back of the
laboratory. Make sure the stir bar does not fall into the waste container.
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