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
Chemicals
stirring rod
deionized water
100 mL beaker
graduated cylinder
1.0 M NaCl (aq)
conductivity probe
1.0 M CaCl2 (aq)
droppers for the
1.0 M AlCl 3 (aq)
salt solutions
(salt solutions in dropper bottles may
be provided)
Graphical Analysis Program-Excel
Procedure
1. 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.
2. Rinse the conductivity probe with deionized water from a wash bottle.
3. 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.
4. Monitor the conductance of the deionized water on the screen, until the number
stabilizes. Record the conductance (S).
5. Lower the beaker away from the probe and add one drop of 1.0 M NaCl. Stir with
a clean stirring rod, raise the beaker to the probe, swirl briefly and 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.
6. Repeat procedure #5, using 8 drops of 1.0 M CaCl2 to fresh deionized water and then a
total of 8 drops of 1.0 M AlCl3 to yet another sample of deionized water.
Measure the conductance after each addition of a drop.
7. Graph the specific conductance (y axis) vs. the # of drops of solutions added (x-axis). If
doing this by hand, it is possible to do all three plots on one paper. If graphing by Excel, it
may be easier to do three separate plots. Your instructor will advise you whether to use the
Excel program or do the graph by hand.
Instructions for using the Excel program can be found in this manual in the
Introduction.
8. Determine the slope of each plot, and the equation of each line, in the form of:
y = mx + b
where y = the specific conductance, m = the slope, x = the # of drops, and b is the y
intercept. ( b will be a graphical average of the conductance of the deionized water,
based on all the other measurements).
Note: If plotting this by hand, use graph paper with divisions of 5 squares/ cm or
10 squares/ inch, or smaller.
9. Discovery: How about the sizes of the drops from the various bottles? For some extra
credit as your instructor allows, – compare the conductance of solutions made from different
bottles of the same chemical (i.e. two different bottles of CaCl2 ) - make some observations
and draw some conclusions.
EXPERIMENT 9: REPORT
Name: _________________________
CONDUCTANCE OF SALT SOLUTIONS
Section # ____________
Data:
Drops of 1.0 M
solutions
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 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? _____
4. From the graphs, record the slope of each:
Solution:
Slope:
# ions
total ionic charge in solution (both + and -)
NaCl
_________
__ ___
.
2
CaCl2
_________
______
.
4
AlCl3
_________
______
________________
. ( the absolute values of
all the + and – charges)
.
5. Which of the three solutions gave a graph with the greatest slope? ________
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.
6. What is the value of “b,” the “y intercept” in the first experiment ? ______________
( for the NaCl experiment) What is the meaning of the y intercept in this
experiment? (recall: b = y value when x = 0)
7. If you were to compare two solutions, NaCl, and CuSO4 having the same
concentrations, which of the two do you think would exhibit the greater specific
conductance? Explain
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 )