Experiment

Using Conductivity toFind an Equivalence Point

In this experiment, you will monitor conductivity during the reaction between sulfuric acid,H2S04,and barium hydroxide, Ba(OH)2,in order to determine the equivalence point. From thisinformation, you can find the concentration of the Ba(OH)2solution. You will also see the effectof ions, precipitates, and water on conductivity. The equation for the reaction in this experimentis:

Ba2(aq) + 2 0H(aq) + 2 H(aq) + S042(aq) > BaSO(s) + 1-120(1)

Before reacting. Ba(OH)2and H2S04are almost completely dissociated into their respective ions.Neither of the reaction products, however, is significantly dissociated. Barium sulfate is aprecipitate and water is predominantly molecular.

As 0.02 M H2S04is slowly added to Ba(OH)2of unknown concentration, changes in theconductivity of the solution will be monitored using a Conductivity Probe. When the probe isplaced in a solution that contains ions, and thus has the ability to conduct electricity, an electricalcircuit is completed across the electrodes that are located on either side of the hole near thebottom 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 microsiemens per centimeter, or jiS/cm.

Prelab Questions (Make sure this is written in your notebook (after the procedure and beforethe data table) before lafr —you will los.yourprefab question points and conclusion points ftills is not JoneO

Prior to doing the experiment, it is very importantforyou to hypothesize about theconductivity ofthe solution at various stages during the reaction. Wouldyou expect theconductivity reading to be high or low, and increasing or decreasing, in each ofthesesituations? Explain your reason in each case.

• When the Conductivity Probe is placed in Ba(’0H)2,prior to the addition ofH2SO4• AsH2S04is slowly added, producing BaSO4andH20.• When the moles ofH2S04added equal the moles ofBaSO4originally present.• As excess HS04is added beyond the equivalence point.

OBJECTIVES

In this experiment, you will

• Hypothesize about the conductivity of a solution of sulfuric acid and barium hydroxide atvarious stages during the reaction.

• Use a Conductivity Probe to monitor conductivity during the reaction.• See the effect of ions, precipitates, and water on conductivity.

CHOOSING A METHOD

Method 1 has the student deliver volumes of H2SO4 titrant from a buret. After titrant is added,and conductivity values have stabilized, the student is prompted to enter the buret readingmanually and a conductivity-volume data pair is stored.

Chemistry with Computers 26 - 1

Evperiment 26

Method 2 uses a Vernier Drop Counter to take volume readings. H2S04titrant is delivered dropby drop from the reagent reservoir through the Drop Counter slot. After the drop reacts with thereagent in the beaker, the volume of the drop is calculated, and a conductivity-volume data pair isstored.

MATERIALSMaterials for both Method I (buret) and Method 2 (Drop Counter)

computer magnetic stirrerVernier computer interface stirring bar or Vernier MicrostirrerLoggerPro 100 mL graduated cylinderVernier Conductivity Probe Phenolphthalein60 mL of—0.02 M H2S04 ring stand50 mL of Ba(OH)2,unknown concentration 1 utility clamp250 beaker

Materials required only for Method I (buret)50 mL buret 2nd utility clamp2nd 250 rnL beaker

Materials required only for Method 2 (Drop Counter)Vernier Drop Counter 150 mL beaker60 mL reagent reservoir 10 mL graduated cylinder25 mL pipet and pipet filler

Measuring Vcrume tising a BliretI. Obtain and wear goggles and an apron.

2 Measure out approximately 60 mL of—0 02 M H2S04solution into a 250 mL beaker Recordthe preciseH2S04concentration in your data table CAUTION H2S04is a strong acid andshould be handled with care. Obtain a 50 mL buret and rinse theburet with a few mL of the H2SO4solution. Use a utility clampto attach the buret to the ring stand as shown here. Fill the bureta little above the 0.00 mL level of the buret. Drain a smallamount ofH2S04solution so it fills the buret tip and leaves theH2S04at the 0.00 mL level of the buret. Dispose of the wastesolution from this step as directed by your instructor.

3 Measure out 500 mL of Ba(OH)2solution of unknownconcentration using a 100 mL graduated cylinder. Transfer thesolution to a clean, dry 250 mL beaker. Then add 120 mL ofdistilled water and 2-3 drops of phenolphthalein to the beaker.CAUTION: Ba’OH,, is toxic. Handle it with care.

4. Arrange the buret, Conductivity Probe, beaker containingBa(OH)2 and stirring bar as shown here The ConductivityProbe should extend down into the Ba(OH)2solution to justabove the stirring bar. Set the selection switch on the amplifier0-2000 .tS/cm range.

//

26-2

box of the probe to the

Chemistry with Computers

Ucing Conductivity to Find an Equivalence Point

5. Connect the Conductivity Probe to the computer interface. Prepare the computer for datacollection by opening the file “26a Conductivity Eq Point” from the Oiemistty withComputers folder of LoggerPro.

6 Before adding H2S04titrant click [i Collect and monitor thedisplayed conductivity value(in p.S/cm) Once the conductivity has stabilized click riep 1 In the edit box, type ‘0”, thecurrent buret reading in rnL. Press ENTER to store the first data pair for this experiment.7. You are now ready to begin the titration. This process goes faster if one person manipulatesand reads the buret while another person operates the computer and enters volumes.

a. Add 1.0 mL of 0.02 M H2S04to the beaker. When the conductivity stabilizes, again clickeep 1. In the edit box, type the current buret reading. Press ENTER. You have now savedthe second data pair for the experiment.b. Continue adding 1.0 mL increments ofH2S04,each time entering the buret reading, untilthe conductivity has dropped below 100 p.S/cm Be sure to note the phenoiphathiernendpoint!c. After the conductivity has dropped below 100 p.S/cm, add one 0.5 mL increment and enterthe buret reading.d After this use 2-drop increments (—0 1 mL) until the minimum conductivity has beenreached at the equivalence point. Enter the volume after each 2-drop addition. When youhave passed the equivalence point, continue using 2-drop increments until the conductivityis greater than 50 p.S/cm again.e. Now use 1.0 mL increments until the conductivity reaches about 2000 p.S/cm, or 25 mL ofH2S04solution have been added, whichever comes first.

8 When you have finished collecting data, click [ a stop] Dispos. of th. beaker contents asdIrected by your teacher. (Pour the contents of the beaker into th. large funnel/beakerset up in the hood!! Do not pour the contents into the sink!!)

9 An alternate way of determining the precise equivalence point of this titration is to performtwo linear regressions on the data One of these will be on the linear region of dataapproaching the equivalence point, and the other will be the linear region of data followingthe equivalence point. The equivalence point volume corresponds to the volume at theintersection of these two lines.

a. Drag your mouse cursor across the linear region of data that precedes the minimumconductivity reading. Click on the Linear Fit button, .b. Drag your mouse cursor across the linear region of data thatfollows minimum conductivityreading. Click on the Linear Fit button, .c. Choose Interpolate from the Analyze menu. Then move the mouse cursor to the volumereading when both linear fits display the same conductivity reading. This volume reading willcorrespond to the equivalence point volume for the titration.

10. Print copies of the table.

11. Print copies of the graph.

Chemistry will, Computers 26-3

ILvperiment 26

h. Drag your mouse cursor across the linear region of data that follows minimum conductivityreading. Click on the Linear Fit button, i.

c. Choose Interpolate from the Analyze menu. Then move the mouse cursor to the volumereading when both linear fits display the same conductivity reading This volume reading willcorrespond to the equivalence point volume for the titration.

14. Print copies of the table.

15. Print copies of the graph.

PROCESSING THE DATA

I. Attach your graph and table in your notebook.

2. Next, prepare a data table like the one below in your notebook.

DATA TABLE

Molarity of H2S04(exact — given at start of class!) M

Volume of H2S04 (phenolphthalein end point) mL

Volume of H2S04 (from graph) mL

From the data table and graph that you printed, determine the volume ofH2S04added at theequivalence point The graph should give you the approximate volume at this point Theprecise volume ofH2S04added can be determined by further examination of the data tablefor the minimum voltage. Record the volume ofH2S04.

3 Calculate moles ofH2S04added at the equivalence point Use the molanty, M, of the H2S04and its volume, in L.

4 Calculate the moles of Ba(OH)2at the equivalence point Use your answer in the previousstep and the ratio of moles of Ba(OH)2andH2S04in the balanced equation. (Stoichiometry!You need your balanced equation here!)

5. From the moles and volume of Ba(OH)2,calculate the concentration of Ba(OH)2,in molIL.

Conclusion

Write a conclusion which discusses whether or not your hypothesis was correct and explain why,or why not. Be specific in your explanation.

26 - 6 Chemistry with Computers

SAMPLE RESULTSVolume ConductivityVolume

mL0.01.02.03.04.05.06.07.08.09.0

10.011.0

Volume Conductivity

mL12.013.013.513,613.713.8

ConductivityLS

967.9907.3835.1748.4677.3606.2533.0461.9391.9324.0252.9183.9

‘Is117.055,329.220.816.613.5

L_13.9 12.414.0 13.5

mL14.515.016.017.018.019.020.021.022.023.024.025.0

38.664.7

160.9257.1351.1451.5539.3627.1712.8804.8888.4968.9

14.114.214.314.4

14.516.625.031.3