Using Conductivity to Find an Equivalence Point

Experiment Using Conductivity to Find an Equivalence Point 24 PRE LAB DISCUSSION In this experiment, you will monitor conductivity during the react...
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Experiment

Using Conductivity to Find an Equivalence Point

24

PRE LAB DISCUSSION In this experiment, you will monitor conductivity during the reaction between sulfuric acid, and barium hydroxide in order to determine the equivalence point. From this information, you can find the concentration of a barium hydroxide solution. Before reacting, barium hydroxide and sulfuric acid are almost completely dissociated into their respective ions. Neither of the reaction products, however, is significantly dissociated. Barium sulfate is a precipitate and water is predominantly molecular. Read and answer Pre Lab Question #1a As a 0.0200 M H2SO4 is slowly added to Ba(OH)2 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 (see Figure 1). This results in a conductivity value that can be read by the interface. The unit of conductivity used in this experiment is the microsiemens, or µS. You will also use a Vernier Drop Counter to take volume readings. H2SO4 titrant is delivered drop by drop from the reagent

reservoir through the Drop Counter slot. After the drop reacts with the reagent in the beaker, the volume of the drop is calculated, and a conductivity-volume data pair is stored. Look at Figure 1 below. Read and answer Pre Lab Question #1b and 1c In addition, you will capture the precipitate, and measure its mass. You will have two methods, therefore, of calculating the molar concentration of a barium hydroxide solution that is titrated with a sulfuric acid solution of known concentration (via the standardization process).

Figure 1

MATERIALS

Magnetic stirrer(and stir bar), LabQuest 2, 25 mL pipet, Vernier Conductivity Probe and Drop Counter, distilled water, 100 mL of a 0.0200 M H2SO4, ring stand, 100 mL of Ba(OH) 2 solution with an unknown concentration, 250 beaker , filter paper, funnel, analytical balance, drying oven.

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Experiment 24 PROCEDURES 1. Write a set of procedures and make a data table to standardize a 0.0200 M H2SO4 solution (with your previously 0.1 M NaOH solution from Lab 23) and then calculate and record the precise H2SO4 concentration in your data table. This part of the lab must have its own set of procedures, data tables and calculations. This part of the lab will be called Lab 24B and must be done before doing another procedure in this lab. 2. Obtain a 50 mL buret and clamp. 3. Fill the buret with

0.0200 M H2SO4 using proper buret filling techniques.

4. Calibrate the Drop Counter so that a precise volume of titrant is recorded in units of

milliliters. a. Choose Calibrate from the Sensors menu and select Drop Counter. b. Place a 10 mL graduated cylinder directly below the slot on the Drop Counter,

lining it up with the tip of the reagent reservoir. c. Slowly turn stop cock of the buret so that drops are released at a slow rate (~1

drop every two seconds). You should see the drops being counted on the screen. d. When the volume of H2SO4 solution in the graduated cylinder is between 9 and 10

mL, turn stop cock of the buret to the off position. e. Enter the precise volume of H2SO4 in the graduated cylinder and tap Keep. Discard the H2SO4 solution in the graduated cylinder down the sink 5. Assemble the apparatus. a. Place the magnetic stirrer on the base of the ring stand. Insert the Conductivity

Probe through the large hole in the Drop Counter. b. Attach the Microstirrer to the bottom of the pH Sensor. Rotate the paddle wheel of

the Microstirrer, and make sure that it does not touch the bottom of the Conductivity Probe. c. Adjust the positions of the Drop Counter and reagent reservoir so they are both

lined up with the center of the magnetic stirrer. d. Measure out 25 mL of Ba(OH)2 of unknown concentration using a 25.000 mL pipet. Transfer the solution to a clean 250-mL beaker. Then add approximately 80-100 mL of distilled water to the beaker. CAUTION: Ba(OH)2 is toxic. Handle it with care. e. Lift up the Conductivity Probe, and slide the beaker containing the Ba(OH)2

solution onto the magnetic stirrer. Lower the Conductivity Probe into the beaker. f.

Adjust the position of the Drop Counter so that the Microstirrer on the Conductivity Probe is just touching the bottom of the beaker.

g. Adjust the reagent reservoir so its tip is just above the Drop Counter slot. h. Turn on the magnetic stirrer so that the Microstirrer is stirring at a fast rate.

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Using Conductivity to Find an Equivalence Point 6. You are now ready to perform the titration. 7. Start data collection. No data will be collected until the first drop goes through the Drop

Counter slot. a. Slowly turn stop cock of the buret so that drops are released at a slow rate (~1

drop every two seconds). When the first drop passes through the Drop Counter slot, check the graph to see that the first data pair was recorded. b. Observe that the conductivity readings decrease gradually. Continue watching

your graph to see when the conductivity decreases and then begins to increase; this will be the equivalence point of the reaction. When this increase in conductivity occurs, let the titration proceed for several more milliliters of titrant. c. Stop data collection to view a graph of conductivity vs. volume and turn stop cock

of the buret to the off position. 8. Examine the data on the displayed graph to find the equivalence point; that is, the volume

when the conductivity value reaches a minimum. To examine the data pairs on the displayed graph, select the correct point on the graph that corresponds to the end point or equivalence point. Record the H2SO4 volume in your lab notebook. Print a copy of the graph 9. Filter and measure the mass of the barium sulfate precipitate. a. Use a hot plate to warm the beaker of mixture containing the BaSO4 precipitate.

Warm the solution to near boiling for about five minutes to help flocculate the particles. b. While the mixture is heating, set up a ring stand and ring for the filter funnel.

Measure and record the mass of a piece of fine-grade filter paper and set the paper in the funnel. c. Allow the mixture to cool, and then filter it. The liquid need not be at room

temperature to be filtered. Wash the precipitate out of the beaker with small amounts of distilled water, if necessary. d. Dry the precipitate and filter paper in a drying oven for at least 15 minutes. e. Cool the precipitate and filter paper to near room temperature. Measure and record

the mass of the filter paper and precipitate. Repeat until the mass of the filter paper stay constant to 0.005 g. f.

Heat the precipitate again for five minutes, cool the precipitate, and weigh it.

PROCESSING THE DATA 1. Calculate the molar concentration of Ba(OH)2 using the volumetric method and the gravimetric method. In your conclusion report your results and compare the results of each method.

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Experiment 24

Lab 24-Pre Lab Quiz 1. 50.00 mL of a barium hydroxide solution is pipetted into a 250 mL beaker. The concentration of the barium hydroxide solution is unknown, but the 50.00 mL of solution is titrated with a 0.0200 M sulfuric acid solution and the electrical conductivity of the solution is measured as the titration proceeds. The data obtained is plotted on a graph of conductivity vs. volume of sulfuric acid titrated. This graph is shown below. The arrow is pointing to the x and y values when the conductivity was the lowest during the titration.

Conductivity, μS

mL of H2SO4

a. Write a balanced chemical equation for the chemical reaction that occurs in the beaker during the titration. Include states of matter with all substances involved.

b. Explain why the conductivity decreases, passes through a minimum, but never reaches zero conductivity, and then increases as the volume of H2SO4 added to the barium hydroxide is increased.

c. Calculate the molarity of the barium hydroxide the solution.

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Using Conductivity to Find an Equivalence Point

Experiment 24: Using Conductivity to Find an Equivalence Point Name: _____________________________

Item Name & Partner Date Experiment Started Title of Experiment Appropriate Purpose Procedure for Standardizing Sulfuric Acid Pre Lab Quiz Procedure for LabQuest (may write see handout) Graph (with Proper Labels) Calculation of [Ba(OH)2] by volumetric Calculation of [Ba(OH)2] by gravimetric Conclusion Signature TOTAL

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Points/Out of /1 /1 /1 /4 /9 /12 /1 /10 /10 /10 /5 /1 /65

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