Name Date Class

Conductivity

The solubility of a substance describes the ability of one substance to dissolve in another substance. Water is used as a solvent to determine the solubility of various solids. Some substances dissolve in water; some substances do not.

When dissolved in water, the ions in an ionic compound separate, or dissociate. As the ions dissociate, electrons are free to move about in the solution. As these electrons move, it is possible for them to carry an electric current.

In this activity, the conductivity of substances is used to determine to what extent substances dissociate in water. This lab compares an ionic solid to a nonionic substance and relates the effect of concentration on conductivity.

Problem

How can an electric current be used to determine the extent to which solid substances dissolve in water?

Objectives

  1. Predict what substances will dissociate in water based on their chemical makeup.
  2. Write balanced equations for the dissociation of substances in water.
  3. Compare the conductivity of various solutions.
  4. Classify substances as nonionic or ionic.
  5. Determine what effect concentration has on conductivity.

Materials

CBL unit

Tl graphing calculator

computer

link cable

conductivity probe

adapter cable

dropper bottles (3) with:

  1. 1M NaCI
  2. 1M MgCI2
  3. 1M AICI3

50-mL beakers (2) with:

  1. NaCI
  2. sugar
     (C12H22O11)

400-mL beaker

150-mL beakers (3)

100-mL graduated cylinder

glass stirring rod

ring stand with test-tube clamp

wash bottle of distilled water

microspatula

electronic balance

weighing dishes (2)

plastic beral pipettes (3)

Safety Precautions

Inline Figure

  1. Always wear safety goggles and a lab apron.
  2. Use caution when working with electricity.
  3. The conductivity probe is fragile. Use caution when setting this up in the ring stand.
  4. Never taste any chemical substances.

Pre-Lab

  1. Read over the entire laboratory activity. Write balanced chemical equations for the dissociation of NaCl, MgCl2 and AlCl3 in water. Form a hypothesis as to which of these compounds would conduct the most electricity and the least electricity. Record your hypothesis on the next page.
  2. Which of the following substances would be considered ionic? Which would be nonionic? Explain your reason for each answer.

    1. potassium chloride (KCl)
    2. methanol (CH3OH)
    3. glucose (C6H12O6)
    4. hydrochloric acid (HCl)
    5. zinc oxide (ZnO)

  3. Sketch a diagram of NaCl dissolving in water.

Procedure

Part A: Preparing the CBL System

If your teacher has the CBL system set up, you may skip to Part B.

  1. Set up a ring stand, clamp, and CBL probe as illustrated in Figure A.

    figure A

  2. Plug the conductivity probe into the adapter cable in channel 1 of the CBL unit.
  3. Connect the CBL unit to the graphing calculator with a link cable.
  4. Turn on the CBL unit and the graphing calculator. Choose ChemBio from the list of programs. Press ENTER on the calculator twice.
  5. Choose SET UP PROBES from the MAIN MENU. Enter 1 as the number of probes. On the SELECT PROBES menu, choose CONDUCTIVITY. Enter 1 as the channel number. Then select USE STORED from the CALIBRATION menu and select H 0-2000 MICS from the CONDUCTIVITY menu. Make sure the switch on the box is set to the same value.
  6. From the MAIN MENU, select COLLECT DATA. On the DATA COLLECTION menu, select TRIGGER PROMPT. Allow the unit to warm up and then press ENTER.

Part B: Comparing Ionic Compounds

  1. Label three beral pipettes—NaCl, MgCl2, and AICI3. Fill one of the pipettes with the l.0M solution of NaCl. Fill the other two pipettes with 1.0MMgCl2 and AlCl3, respectively.
  2. Using the 100-mL graduated cylinder, measure 70 mL of distilled water into the 150-mL beaker. Raise the beaker until the conductivity probe is in the water. After the conductivity meter stabilizes, press TRIGGER on the CBL unit.
  3. Measure and record the conductivity of the distilled water in Data Table 1.
  4. Lower the beaker and place 1 drop of NaCl solution into the distilled water. Stir with the glass stirring rod and then raise the beaker until the conductivity probe is in the solution. After the conductivity meter stabilizes, press TRIGGER on the CBL unit. Measure and record the conductivity of this solution in Data Table 1.
  5. Adding 1 more drop of the NaCl solution, repeat step 4. Continue adding 1 drop and recording its conductivity until a total of 8 drops of NaCl solution has been added.
  6. If a TI-83 graphing calculator is being used, or another type of graphing calculator and a computer is available, refer to Appendix A for instructions on how to convert this data into graphical analysis.
  7. After transferring the data to the graphing program, rinse out the beaker with distilled water and repeat steps 4–6 two more times using MgCl2 and AlCl3, respectively. Be sure to transfer the data to the graphing program and rinse out the beaker after each substance. Rinse the conductivity probe with distilled water in between each substance. (Do this by spraying the probe over the 400-mL beaker or sink.)

Part C: Comparing Ionic and Molecular Substances

  1. In two separate weighing dishes, measure 10 g of sodium chloride (NaCl) and 10 g of sucrose (C12H22O11).
  2. Using the 100-mL graduated cylinder, place 50 mL of distilled water in each of two 150-mL beakers. Label one of the beakers sodium chloride (NaCl) and the other beaker sucrose (C12H22O11).
  3. Pour the solid sodium chloride and sucrose into the appropriate beakers and stir with a glass stirring rod.
  4. Use the conductivity probe to monitor the conductivity of the sodium chloride solution. Record the conductivity in Data Table 2.
  5. Rinse the probe with distilled water.
  6. Use the conductivity probe to monitor the conductivity of the sucrose solution. Record the conductivity in Data Table 2.

Cleanup and Disposal

  1. Disconnect the conductivity probe from the CBL unit.
  2. Rinse the probes with distilled water.
  3. Rinse out the beakers with distilled water.
  4. Clean up your lab area and wash your hands. Replace the lab equipment to the appropriate area.

Hypothesis



Data and Observations

Data Table 1 (to be used if graphing program not available)
Drops NaCl conductivity
(microsiemens) 		MgCI2 conductivity
(microsiemens) 		AICI3 conductivity
(microsiemens)
0      
1      
2      
3      
4      
5      
6      
7      
8      

Data Table 2
Substance Conductivity
Sodium chloride (NaCl)  
Sucrose (C12H22O11)  

Analyze and Conclude

  1. Observing and Inferring Which solution was the best conductor of electricity? Explain.



  2. Making and Using Graphs Make a graph of conductivity versus concentration. Plot conductivity on the y-axis and concentration (number of drops) on the x-axis. If you used a graphing program, you may use those graphs. Draw a line of best fit for each of these sets of data.





















  3. Observing and Inferring For each of the dissociation reactions of NaCl, MgCl2 and AlCl3, what is the ratio of number of electrons transferred in each reaction? How does this explain the graphs drawn in question 2?




  4. Drawing a Conclusion How does the conductivity of sodium chloride compare with sucrose? Why is this the case?



Real-World Chemistry

What types of substances make good conductors of electricity? What element is used to bring electricity to the places where we live? Why is this an effective substance?