Experiment

18

Investigating Chemistry through Inquiry 18 - 1 S

PRELIMINARY ACTIVITY FOR

Conductimetric Titrations Open Inquiry Version

There are a number of ways to investigate a chemical reaction. Measuring temperature change, pressure change, pH change, and/or conductivity change are some of the ways. A titration is a process used to determine the volume of a solution needed to react with a given amount of another substance. You have probably done pH titrations in other experiments; now you will be performing conductivity titrations. In the Preliminary Activity, you will titrate a solution of the strong acid hydrochloric acid, HCl, with a solution of the strong base sodium hydroxide, NaOH. The concentration of the NaOH solution is given, and you will determine the unknown concentration of the HCl. Hydrogen ions from the HCl react with hydroxide ions from the NaOH in a one-to-one ratio to produce water in the overall reaction:

H+(aq) + Cl–(aq) + Na+(aq) +OH–(aq) ⎯ → ⎯ H2O(l) + Na+(aq) + Cl–(aq)

As 1.00 M NaOH is slowly added to HCl of unknown concentration, changes in the conductivity of the solution will be monitored using a Conductivity Probe. When the probe is placed in a solution that contains ions, and thus has the ability to conduct electricity, an electrical circuit is completed across the electrodes that are located on either side of the hole near the bottom of the probe body. This results in a conductivity value that can be read by the interface. The unit of conductivity used in this experiment is the microsiemens/cm, or µS/cm.

At the equivalence point, equimolar amounts of HCl and NaOH have been mixed. When an HCl solution is titrated with an NaOH solution, the conductivity of the acidic solution is initially high. As NaOH is added prior to the equivalence point, conductivity does not increase—it decreases instead, as shown in Figure 1, because added OH– ions react with H+ to form H2O and because the Na+ ions replacing the high conductivity H+ ions in solution have much lower conductivity.

Figure 1

Evalua

tion co

py

Experiment 18

18 - 2 S Investigating Chemistry through Inquiry

Once the equivalence point has been reached, conductivity increases because Na+ and OH– ion concentrations increase as further titrant is added.

You will determine the volume of NaOH titrant used at the equivalence point using the intersection of the linear fits of the two branches of the resulting curve, as shown above. The volume of NaOH titrant used at the equivalence point will be used to determine the molarity of the HCl.

After completing the Preliminary Activity, you will first use reference sources to find out more about conductimetric titrations before you choose and investigate a researchable question involving conductimetric titrations. Some topics to consider in your reference search are:

• titrations • conductimetric titrations • strong and weak acids • strong and weak bases

• equivalence point • neutralization • precipitates • precipitation reactions

PROCEDURE 1. Obtain and wear goggles.

2. Use a pipet to add 10.00 mL of a hydrochloric acid solution of unknown concentration to a 250 mL beaker. Dilute to about 100 mL using distilled water. CAUTION: Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with the skin.

3. Place the beaker on a magnetic stirrer and add a stirring bar. If no magnetic stirrer is available, stir the reaction mixture with a stirring rod during the titration.

4. Set up the data-collection system.

a. Connect the Conductivity Probe and the data-collection interface.

b. Start the data-collection program. c. Set up data collection for Events with Entry mode.

5. Use a utility clamp to suspend the Conductivity Probe on

the ring stand (see Figure 2). Position the Conductivity Probe so that its tip is immersed in the HCl solution but is not struck by the stirring bar. Gently stir the beaker of acid solution.

6. Rinse and fill a 50.0 mL buret with ~1.00 M NaOH solution. Attach the buret to the ring stand using a buret clamp (see Figure 2). Note: Record the precise concentration of the NaOH solution in your data table. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.

Figure 2

Conductimetric Titrations

Investigating Chemistry through Inquiry 18 - 3 S

7. Conduct the titration carefully. Listed below is a suggested method for performing a conductimetric titration.

a. Begin data collection, and then determine an initial conductivity reading before adding any NaOH solution. When the reading stabilizes, select Keep, enter 0 (the current buret reading), and then select OK.

b. Add 0.5 mL of the NaOH solution to the beaker. When the reading stabilizes, select Keep, enter the buret reading, and then select OK.

c. Continue adding 0.5 mL increments of NaOH until you have approximately the same number of readings beyond the equivalence point as before it, and then stop data collection. In Figure 1 above, the equivalence point is at the intersection of the two branches.

8. When you have finished the titration, dispose of the reaction mixture as directed. Rinse the

Conductivity Probe with distilled water.

QUESTIONS 1. Use the analysis tools of your data-collection program to determine the volume of NaOH

added at the equivalence point. Record the volume in the space below.

Alternatively, add the volume of NaOH corresponding to the lowest point on the left branch of your graph and the volume of NaOH corresponding to the lowest point on the right branch, and then divide by 2.

2. Calculate the number of moles of NaOH used.

3. Using the equation for the neutralization reaction given in the introduction, determine the number of moles of HCl used.

4. Calculate the HCl concentration using the measured volume of unknown HCl.

5. List at least one researchable question concerning conductimetric titrations.

Note: The plan that you submit for instructor approval should list laboratory safety concerns, including chemical safety concerns, and specify how you will address these safety concerns during your investigation.

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Safety information Essential instructor background information Directions for preparing solutions Important tips for successfully doing these labs

The complete Investigating Chemistry though Inquiry lab manual includes 25 inquiry-based labs and essential teacher information. The full lab book is available for purchase at: http://www.vernier.com/cmat/chemi.html

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