EXPERIMENT 9 - Faculty Web Pages

EXPERIMENT 9
SALTWATER CONDUCTANCE: The Effect of Concentration
Introduction
According to the “Theory of Ionization” proposed by S. Arrhenius, about 1880, ionic compounds
dissolve in water forming cations and anions. Such a solution acts as an “electrolyte”, conducting
electricity. Arrhenius determined that “cations” moved toward the negatively charged cathode in a
battery-type set-up. Thus cations were determined to carry positive charge, and “anions,” which
moved toward the positively charged anode, were determined to have negative charge. He found
that the amount of current conducted by the solution depended on the number of ions in solution
and the charges on the ions.
NaCl (s)
Na+ (aq) + Cl¯ (aq)
Conductivity can be measured (in units of S, microsiemens) using a probe connected to a meter. In
this experiment you will study the effect of increasing the concentration of an ionic compound, on
the specific conductance. The concentrations of the solutions will gradually be increased by adding
drops of more concentrated solutions to the beakers and the conductance will be measured.
Materials needed
Figure 1
Chemicals
stirring rod
deionized water
(3) 100 mL beaker
graduated cylinder
1.0 M NaCl (aq)
conductivity probe
1.0 M CaCl2 (aq)
plastic dropper
1.0 M AlCl3 (aq)
Graphical Analysis Program-Excel
Venier LabQuest & conductivity probe
In Part 1: the student will determine which toggle setting is most appropriate for this experiment.
The actual units of conductivity used are called “specific conductance” - the conductivity across 1
cm of solution between two electrodes in a probe measured in “microSiemens” or S. The toggle
switch settings allow greater precision at low levels of conductance.
Toggle switch
settings =>
0-20000 - use this setting for conductance above about 2500 up to 30000
0-200 - use this for conductance below abut 250
0-2000 - use this for mid-range values of 300 – 2500.
Note – “Baseline” readings are subtracted from all readings. Different baseline readings will be
shown at the different settings – for example at the high setting, the baseline reading for DI water
may be 100, but at the low setting – maybe 10. Rinse the probe in DI water between each reading.
Keep a large beaker of DI water handy. Students should work in pairs due to the number of beakers
needed. In Part 2: the student will compare the conductance of three different salt solutions at
similar concentration. Then in Part 3: the student will correlate concentration to conductance by
doing serial dilution of one of the salts and measure the respective conductance.
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Procedure Part 1.
Obtain a large Florence flask and fill with DI water. Clean and rinse all your beakers and flasks in
deionized water. Fill the 250 or 100 mL beaker with 70.0 mL of deionized water, insert the
conductance probe and record the “baseline” conductance for each of the toggle switch settings.
Now place 10 drops of AlCl3 salt solution into the beaker of DI water, mix well, then re-measure
the conductance at each toggle switch. Compare the high and low values to determine which setting
would be most appropriate for this experiment. Remember, if you are using the 0-250 S setting
and you read a value of around 260 but on the other settings the values read in the thousands, then
that setting in inappropriate since it cannot measure the entire range.
Procedure Part 2.
Carefully clean and rinse a 100 mL beaker with deionized water. Also clean and prepare a
graduated cylinder. Measure 70.0 mL of deionized water into the beaker, using a graduated
cylinder.
Rinse the conductivity probe with deionized water from a wash bottle. If you are using the
Venier’s Labquest Conductivity probe; set the selector switch on the side of the Conductivity Probe
to the µS/cm range determined from Part 1. Connect the Conductivity Probe to LabQuest and then
choose New from the File menu. Submerge the probe in the beaker containing the 70.0 mL of
water, as shown in Figure 1, such that the holes are completely under the water. The electrodes in
the probe are on either side of these holes, so it is vital that the holes be under the level of the water.
Once the number on the screen stabilizes, record the conductance (S) on your report sheet.
If Dropper Bottles of the solutions are not provided read this section, if dropper bottles are
provide skip to the next paragraph. Obtain a clean medicine dropper (plastic disposable dropper)
and rinse several times with deionized water. Using your 10 mL graduated cylinder, count how
many drops of deionized water from the dropper is required to fill the graduated cylinder to 1.0 mL,
exactly. Repeat to verify your findings. You have calibrated your dropper. For consistency, you will
use this same dropper for all three solutions so remember to clean it well with deionized water
between each trial. Avoid cross contamination.
In your beaker containing 70.0 mL DI water squeeze one drop of the 1.0 M NaCl solution, swirl
to mix well, then record the conductance. Continue this procedure, recording the conductance after
each drop of 1.0 M NaCl has been added, until a total of 8 drops has been added. Be sure to rinse
the probe each time with deionized water from a wash bottle.
Dispose of the NaCl solution, clean both beakers and the dropper then repeat the above
procedure using CaCl2, adding up to 8 drops of 1.0 M CaCl2 to fresh deionized water. Measure the
conductance after each addition of a drop. Dispose of the CaCl2 solution, clean both beakers and the
dropper then repeat the same experiment with AlCl3 (8 drops) and fresh deionized water. Measure
the conductance.
Discovery: Is the result affected by the differing sizes of the drops from the various bottles?
Compare the conductance of solutions made from different bottles of the same chemical (i.e. 2
different bottles of CaCl2). Make some observations and draw some conclusions.
Procedure Part 3.
In this section, you will learn to make a series of dilutions. Clean and re-use flasks/beakers
for the following procedure. Record the conductance of the 4 solutions made below using the (020000) setting. Make sure to rinse the probe between each reading.
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Pour 20 mL of 0.10 M NaCl into a beaker. Label the beaker 0.10 M NaCl. Transfer 10.0
mL of this 0.10 M solution into a 50 mL graduated cylinder. Use a dropper to get the meniscus
precisely to the mark. Fill the cylinder to the 50.0 mL mark with DI water and pour this mixture
into a new clean beaker or flask. You now have a container with 50.0 mL of 0.020 M NaCl. Label
the container: “”0.020 M NaCl”. This technique is known to as “DILUTION”. The mathematical
equation used to determine the new concentration is M1V1 = M2V2 (the dilution equation). In this
dilution, M1 is 0.10M, V1 is 10.0 mL, and V2 is 50.0 mL.
M1V1 = M2 therefore
V2
(0.10M) (10.0 mL) = M2
(50.0 mL)
therefore M2 = 0.020M
Next, transfer exactly 10.0 mL of this 0.020M NaCl solution into the 50 mL graduated cylinder,
once again using the dropper pipet to read the meniscus precisely, fill to the 50 mL mark with DI
water, and then pour this new solution into another clean container (beaker or flask). Use the
dilution equation to determine the new concentration of the solution and label that container.
Repeat this dilution process one more time (10 mL of “X” diluted to 50 mL). You should now have
four distinct concentrations of NaCl solution in four different containers. Use a single 100 or 50 mL
beaker and measure the conductance of each solution. Think about which order you should
measure the solutions.
Analysis: Calculations and Graphing
Part 2: Graph the specific conductance (y axis) vs. the # of drops of solutions added (x-axis).
Your instructor will advise you whether to use the Excel program or do the graph by hand. Note: If
plotting this by hand, use graph paper with divisions of 5 squares/ cm or 10 squares/ inch, or
smaller. Instructions for using the Excel program can be found in this manual in the Introduction.
Determine the slope of each plot, and the equation
of each line, in the form of:
y = mx + b
S
conductance
MgCl2
where y = the specific conductance in μS, m = the
slope, x = the # of drops, and b is the y intercept (and
represents the conductance of deionized water with no
salt added, or the conductance of the deionized water
itself). Plot all three graphs on the same paper.
Take the slope of each line.
# of drops
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Part 3: Plot the conductance of the NaCl solution
(y axis) vs the concentration (M) (x axis).
Determine its slope. Compare the equation from
Part 2 to the equation in Part 3. How are they
similar or different?
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EXPERIMENT 9: REPORT
Name: _________________________
CONDUCTANCE OF SALT SOLUTIONS
Section # ____________
Data: Part 1
Which setting did you choose? _________________
Part 2
Drops of 1.0 M
solution
Specific Conductance (S)
NaCl
CaCl2
AlCl3
0
1
2
3
4
5
6
7
8
Questions
1. Describe the appearance of each of the three graphs. Are they linear? Attach the graphs (one
graph with all 3 lines on the same axes and one graph from part 3) to this report sheet.
2. Write the chemical equations for the dissociation of CaCl2 and AlCl3, similar to the one for NaCl
that was given in the introduction.
3. How many ions are obtained from dissociation of: NaCl? _____
How many ions are obtained from dissociation of: CaCl2? _____
How many ions are obtained from dissociation of: AlCl3? _____
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4. From the graphs, record the slope of each:
Solution:
Slope:
# ions
total ionic charge (sum of the absolute values of all charges)
NaCl
_________
______
______________
CaCl2
_________
______
______________
AlCl3
_________
______
______________
Which of the three solutions gave a graph with the greatest slope? ________
5. Is there any relationship between the number of ions in solution and the slope of the graph? Or
does the conductance more closely approximate the ratios of the total ionic charges? Explain.
Part 3.
Serial
dilution
Concentration of NaCl
solution
Conductance (S)
0
0.10 M
1
0.020 M
2
3
6. Discuss the similarities and differences between Part 2 and Part 3 NaCl graphs.
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7. What is the value of “b,” the “y intercept” in the Part 2 & 3 (for the NaCl experiment)?
What is the meaning of the y intercept in this experiment? (Recall: b = y value when x = 0)
8. From the graph or equation, predict what the specific conductance of a solution of AlCl3 in your
experiment would be, after adding 16 drops of the 1.00 M AlCl3 to the 70.0 mL of deionized water.
(You may have to use the equation of the line to calculate it, if the graph is too small to extend so
far.)
9. Predict the concentration of NaCl if the conductance read 245 S.
(You may have to use the equation of the line to calculate it, if the graph is too small to extend so
far.)
10. If you were to compare two solutions, one of NaCl, and one of CuSO4 having the same
concentration, which do you think would exhibit the greater specific conductance? Explain.
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