Conductivity Experiment

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 look at the conductance as three brine (salt) solutions are formed. Those solutions are 0.5 M NaCl, 1.0 M NaCl and 0.5 M CaCl₂.

PROCEDURE

1. Obtain and wear goggles.

2. Get 50 mL of distilled water in a clean 100-mL beaker.

3. Prepare the conductivity probe for data collection.

• Plug the conductivity probe into the adapter cable in Channel 1 of the CBL.
• Connect the CBL System to the TI-8X calculator with the link cable using the port on the bottom edge of each unit. Firmly press in the cable ends.

4. Turn on the CBL unit and the TI-8X calculator. Press PRGM and select CHEMBIO. Press ENTER, then press ENTER again to go to the CHEM MAIN MENU.

5. Set up the calculator and CBL for a conductivity probe.

• Select SET UP PROBES from the CHEM MAIN MENU.
• Enter "1" as the number of probes.
• Select CONDUCTIVITY from the SELECT PROBE menu.
• Enter "1" as the channel number.
• Select USE STORED from the CALIBRATION menu.
• Select 0-100 MG/L

6. Set up the calculator and CBL for data collection.

• Select COLLECT DATA from the CHEM MAIN MENU.
• Select TRIGGER/PROMPT from the DATA COLLECTION menu.

7. Assemble a conductivity probe, utility clamp, and ring stand. Be sure the electrodes are clean and dry before beginning the experiment.

8. Before adding any salt solution

• Raise the beaker until the probes touch the bottom of the beaker.
• Monitor the conductivity of the distilled water displayed on the CBL screen for 3-4 seconds.
• Press TRIGGER on the CBL, and then enter "0" (the volume, in drops). The conductivity and volume values have now been saved for the first trial. This gives the relative conductivity of the water before any salt solution is added.
• Lower the beaker away from the probes.
• Record the conductivity value in your data table.
• Click on the OK button. The relative conductivity value and NaCl drops are now saved.

9. You are now ready to begin adding salt solution.

• Select MORE DATA.
• Add 1 drop of salt solution to the distilled water.
• Swirl to ensure thorough mixing.
• Raise the beaker until the probes touch the bottom of the beaker.
• Monitor the conductivity of the solution for 3-4 seconds.
• Press , and then enter "1" (the volume, in drops). The conductivity and volume values have now been saved for the second trial.
• Lower the beaker away from the probes.
• Record the conductivity value in your data table.
• Click on the OK button.

10. Repeat the Step 9 procedure, entering "2" this time. Record the conductivity value in your data table.

11. Continue this procedure, adding 1-drop portions of salt solution, measuring conductivity, and entering the total number of drops added&emdash;until a total of 10 drops have been added.

12. STOP AND GRAPH from the DATA COLLECTION menu when you have finished collecting data. Use TRACE to examine the data points along the displayed graph of relative conductivity vs. volume. As you move the cursor right or left, the volume (X) and relative conductivity (Y) values of each data point are displayed below the graph. Confirm the relative conductivity and volume data pairs you recorded in your data table.

13.
TI-83, TI-85 Use the STAT EDIT menu to rename the list containing the conductivity values.

TI-82 Use the STAT EDIT menu to view the list containing the conductivity values, record these values on a seperate sheet of paper.

14. Repeat the titration with a new 50-mL distilled water sample and the 1.0M NaCl solution. Move to Procedure #4.

15. Repeat the titration with a new 50-mL distilled water sample and the 0.5 M CaCl₂ solution. Move to Procedure #4.

16. Dispose of the beaker contents as directed by your instructor.

Drops		Conductivity
	0.5 M NaCl	1.0 M NaCl	0.5 M CaCl2
0	0.184	0.184	0.184
1	0.338	0.735	0.552
2	0.552	1.10	0.919
3	0.735	1.47	1.29
4	0.919	1.84	1.47
5	1.10	2.02	1.84
6	1.29	2.21	2.21
7	1.47	2.39	2.21
8	1.47	2.57	2.39
9	1.65	2.74	2.39
10	1.84	2.94	2.57

Questions

1. Construct a plot of conductivity vs. NaCl concentration (or NaCl volume, in drops) using your data. hint: use three list and two stat plots for two experiments.

2. Describe the appearance of the curve on your graph.

3. Describe the change in conductivity as the concentration of the NaCl solution was increased by the addition of NaCl drops. What kind of mathematical relationship does there appear to be between relative conductivity and concentration?

4. What causes the difference in slope between the two lines?

5. Construct a plot of conductivity vs. CaCl₂ concentration. hint: use a fourth list and a previous list.

6. How is this plot similar/different from the previous NaCl plots.

7. Construct a plot of conductivity vs. NaCl concentrations and CaCl₂ concentrations using your data. hint: use four lists and three stat plots for all three experiments.

8. Explain what you see from a mathematical and chemical perspective.