Chemistry Lab #3:
Conductivity of Solutions: The Effect of Concentration
Background
If an ionic compound is dissolved in water, it dissociates into ions and the resulting solution
will conduct electricity. Dissolving solid sodium chloride in water releases ions according to the
equation:
NaCl (s) —> Na+ (aq) + Cl– (aq)
In this experiment, you will study the effect of increasing the concentration of an ionic
compound on conductivity. Conductivity will be measured as concentration of the solution is
gradually increased by the addition of concentrated NaCl drops. The same procedure will be
used to investigate the effect of adding solutions with the same concentration (0.1 M), but
different numbers of ions in their formulas: calcium chloride, CaCl2 and sodium phosphate,
Na3PO4. A computer-interfaced Conductivity Probe will be used to measure conductivity of the
solution. Conductivity is measured in microsiemens per centimeter (μS/cm)
Conductivity
The conductivity probes you will use in this lab activity measure how well a solution
conducts electricity. It gives a low reading for a low concentration of ions and a higher reading
for higher concentrations of ions. In effect, the reading produced by the probe is a measure of
the number of ions per liter. The number of ions per liter depends on two things. First, it
depends on how much solid is dissolved. The more material that is present in solution, the more
conductive the solution will be. Second, it depends on the chemical formula of the dissolved
material. Some compounds have more ions in their chemical formulas than others. Consider the
following compounds and the number of ions each one makes when they dissolve:
sodium chloride
NaCl —> Na+ + Cl–
calcium chloride CaCl2 —> Ca2+ + Cl– + Cl–
sodium phosphate Na3PO4 —> Na+ + Na+ + Na+ + PO43–
Each equation shows how many ions are produced from each formula when they dissolve in
water. Sodium chloride produces two ions, calcium chloride produces three ions, and sodium
phosphate produces four ions. Even when these salts are made into solutions with the same
concentration, measured as whole formula units per liter, they produce different levels of
conductivity. For example, a solution of sodium phosphate will be twice as conductive as a
solution of sodium chloride with the same concentration.
Objectives
In this lab you will:
 Use a Conductivity Probe to measure the conductivity of various solutions
 Investigate the relationship between the conductivity and concentration of a solution
 Investigate the conductivity of solutions resulting from compounds that dissociate to
produce different number of ions.
Questions:
Part I: Which of the five solutions (NaCl, CaCl2, Na3PO4, C12H22O11, or H2O2 )do you predict will
have the highest conductivity? the lowest? Why?
Part II: Which of the three test solutions (NaCl, CaCl2, or Na3PO4) )do you predict will show the
greatest increase in conductivity with increasing concentration (# of drops)? the lowest? Why?
Materials:








Labquest2
Vernier Conductivity Probe
ring stand
clamp
stirring rod
several 100-mL beakers
distilled water
wash bottle with distilled water





100-ml graduated cylinder
dropper bottle of 0.1 M NaCl
dropper bottle of 0.1 M CaCl2
dropper bottle of 0.1 M Na3PO4
dropper bottle of 0.1
M C12H22O11 (sugar)
dropper bottle of
3% H2O2 (hydrogen peroxide)
Sharpie marker for labelling

Safety

The chemicals used in this activity are all of low toxicity and are used in dilute solutions.
Wear your safety goggles while working in the lab.
Part I: Conductivity
Procedure
In this part of the lab you will measure the conductivity of six different substances: distilled
water, hydrogen peroxide (H2O2) solution, sucrose (C12H22O11) solution, sodium chloride (NaCl)
solution, calcium chloride (CaCl2) solution, and sodium phosphate (Na3PO4) solution.
1. Set up the Conductivity Probe using the ring stand and clamp. Do not clamp the probe
too tightly and do not submerge the probe completely into water at any time.
2. Set the range selector on the Conductivity Probe to “0 - 200 μS/cm”.
3. Connect the probe to the LabQuest2 and start the Logger Pro software.
4. The software will immediately recognize the probe and produce a data collection screen.
5. Label six 100-mL beakers or plastic cups with the formulas of the substances you will be
testing: distilled water, NaCl, CaCl2, Na3PO4, C12H22O11, and H2O2. You will use these
beakers/cups throughout this lab without emptying them until it is time to clean up.
6. Fill each beaker/cup with 50 mL of distilled water
7. Test the conductivity of the beaker/cup containing distilled water by submerging the
electrode at the bottom completely. Distilled water alone will be the first material you
test. Hold the beaker/cup up to the probe and submerge the end of the probe in the
water. Record the conductivity value on your screen in Data Table 1.
8. Now add just one drop of each solution your teacher has provided for you to each labeled
beaker.
9. Stir the NaCl solution with the glass rod. Rinse the rod and dry it before stirring the next
solution. Stir each one with the stirring rod, rinsing and drying in between each one.
10. Use the Conductivity Probe to measure the conductivity of the NaCl solution. Record the
value in Data Table 1.
11. Rinse the end of the probe carefully using the wash bottle full of distilled water. This
prevents the probe from giving inaccurate readings when you test the next solution.
12. Now use the probe to measure the conductivity of each of the other solutions. Record the
conductivity values of each in Data Table 1. Always be sure to rinse the probe between
tests!
Part II: Electrolyte Conductivity vs. Concentration
Procedure
In this part of the lab you will collect data to show the proportion between concentration and
conductivity for three strong electrolytes.
1. On the “Experiment/Main” menu, click “Sensors…” choose “Data Collection…”. change
the “Mode:” to “Events with Entry”.
2. For “Name” enter “Volume” and for “Units” enter “Drops”. Click OK
3. Click the “Collect” button to start data collection.
(green arrow)
4. Raise the beaker/cup containing 50 mL distilled water and one drop of NaCl solution up
over the end of the probe. Click the Keep button
and enter 1 for the volume in
drops. Click “OK”
5. Add another drop of NaCl solution and stir it using your clean stirring rod.
6. Test the conductivity of the solution by holding it so the end of the probe is submerged.
Swirl it gently, without spilling and watch the conductivity value for about 5 seconds.
Once the reading reaches a steady value, click “Keep” and enter 2.
7. Repeat this process until you have reached a total of 8 drops.
8. Stop data collection by clicking the “Stop” button.
(red square)
9. Next you will use the software to analyze your result by producing a linear regression.
This will calculate the equation of the line and plot a best-fit line for your data.
a. On the “Analyze” menu choose “Curve Fit”. Under “Choose Fit” select “Linear”.
Click OK
b. When it plots the line and gives you the equation it will be in the form y = mx + b.
The y variable stands for conductivity, the x value stands for volume in drops.
c. Record the value of the slope, m, in Data Table 2.
d. Click the File Cabinet icon to “Store” the data from the first run.
10. Rinse the electrode with the wash bottle.
11. Repeat the data collection procedure with the CaCl2 solution.
12. Remember to store the data from Run 2 using the File Cabinet icon.
13. Repeat the data collection process with the Na3PO4 solution.
14. Make sure you record the conductivity values for the three solutions in Data Table 2.
(Click on the table icon to view your data tables and to record the values for each run) In
your report you must include a well-formatted graph with all three runs properly labeled
and showing the slope of each line.
Data Table 1 __________________________________
Solution
Conductivity (μS/cm)
Data Table 2 ___________________________________
Drops
1
2
3
4
5
6
7
8
Slope, m
Solution: _______
Solution: _______
Solution: _______
Conductivity (μS/cm)
Conductivity (μS/cm)
Conductivity (μS/cm)
Discussion Questions
Answer the following questions, as part of your report you must include your labeled data
tables and graph, and the answers to the questions below. This lab does require a formal lab
report.
Part I:
1. Which of the substances you tested in part one were electrolytes? Which of them were
non-electrolytes? Use your data to justify your answers and recall that distilled water
should have essentially zero conductivity. If your distilled water had a reading slightly
above zero then take this value as equivalent to zero conductivity.
2. Using the chemical formulas of each compound explain why the conductivity values
follow the pattern that you see in Data Table 1.
3. Why do some compounds show essentially zero conductivity?
4. Why are some more conductive than others despite the fact that you added the same
amount of each one?
Part II:
5. What kind of proportion exists between the conductivity of a solution and the
concentration of ions in the solution? Describe what happens to conductivity as
concentration increases.
6. Compare the slopes of the three lines and put them in order from smallest to largest.
What explains the difference in the slopes since all three were generated starting with
solutions with the same starting concentration?
7. Write equations for when the following salts dissolve in water: Na2SO4, Mg3(PO4)2,
and Al(OH)3.
8. Which of the salts in the previous problem would have the steepest slope if it were used
in Part II of this lab? Which would have the smallest slope? Justify your answers.