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Experiment 7Determination of the Ka of a Weak Acid and the Kb of a WeakBase from pH MeasurementsPre-Lab AssignmentBefore coming to lab: Read the lab thoroughly. Answer the pre-lab questions that appear at the end of this lab exercise. The questionsshould be answered on a separate (new) page of your lab notebook. Be sure to show allwork, round answers, and include units on all answers. Background information can befound in Chapter 16 and 17, especially sections 17.1- 17.3 and 16.9 in your textbook(Brown and LeMay). Follow the guidelines in the "Lab Notebook Policy and Format for Lab Reports" section ofthe lab manual to complete in your lab notebook the following sections of the report forthis lab exercise: Title, Lab Purpose, Procedure and Data Tables.PurposeIn this experiment you will learn to calibrate and use a pH probe, and then construct a titrationcurve (graph) in order to determine the molarity and Ka (acid dissociation constant) of anunknown acid. Similarly, you will then titration a weak base (NH3) with a strong acid to calculatethe value of Kb (base dissociation constant) for the weak base. Finally, you will use the pH meterin order to determine the effect of dissolved salts on the pH of water through the process ofhydrolysis and the effect of adding an acid or base to a buffer.BackgroundIn Chem 1a, you performed a titration experiment in order to determine the concentration(Molarity) of an unknown acid. In any titration experiment, an accurately known number of molesof one substance (either the acid or the base) is used to determine the number of moles of theother. The point were moles of acid moles of base is called the equivalence point. In a welldesigned experiment, the equivalence point corresponds to the endpoint of the titration- the pointat which an indicator changes color due to a large change in pH. In chem 1a, you used theindicator phenolphthalein which turned from colorless to pink to indicate the endpoint of thetitration. The concentration of the unknown substance can then be calculated from the dataobtained from the equivalence point, the amount of known substance reacted, and volume of theunknown substance that you titrated. An example of this type of calculation is on the next page ofthe lab.Determination of the Ka of a Weak Acid and the Kb of a Weak Base from pH Measurements1

Example Problem: Concentration Determination By TitrationCitric acid (C6H8O7 or H3C6H5O7) is the primary acid present in many fruits that gives them a sourtaste. Suppose that 18.7 mL of 0.102 M NaOH is required to neutralize 20.0 mL of a citric acidsolution. The equation for neutralization is given below. What was the concentration of thecitric acid solution?3 NaOH (aq) H3C6H5O7 (aq) Β  Β Na3C6H5O7 (aq) 3 H2O (l)Step 1: Moles of starting compound used:1  𝐿0.102 Β π‘šπ‘œπ‘™π‘’π‘  Β π‘π‘Žπ‘‚π»18.7 Β π‘šπΏ Β π‘π‘Žπ‘‚π» Β π‘ π‘œπ‘™π‘’π‘‘π‘–π‘œπ‘› Β  0.0019074 Β π‘šπ‘œπ‘™π‘’π‘  Β π‘π‘Žπ‘‚π»1000 Β π‘šπΏ1  𝐿 Β π‘ π‘œπ‘™π‘’π‘‘π‘–π‘œπ‘›Step 2: Moles of desired compound:1 Β π‘šπ‘œπ‘™π‘’ Β πΆπ‘–π‘‘π‘Ÿπ‘–π‘  𝐴𝑐𝑖𝑑0.0019074 Β π‘šπ‘œπ‘™π‘’π‘  Β π‘π‘Žπ‘‚π» Β  0.0006358 Β π‘šπ‘œπ‘™π‘’π‘  Β πΆπ‘–π‘‘π‘Ÿπ‘–π‘  𝐴𝑐𝑖𝑑3 Β π‘šπ‘œπ‘™π‘’π‘  Β π‘π‘Žπ‘‚π»Step 3: Finish the Problemπ‘Šβ„Žπ‘Žπ‘‘  𝑖𝑠 Β π‘‘β„Žπ‘’ Β π‘šπ‘œπ‘™π‘Žπ‘Ÿπ‘–π‘‘π‘¦ Β π‘œπ‘“ Β πΆπ‘–π‘‘π‘Ÿπ‘–π‘  𝐴𝑐𝑖𝑑 Β π‘†π‘œπ‘™π‘’π‘‘π‘–π‘œπ‘›?1  𝐿𝐿 Β π‘œπ‘“ Β π‘ π‘œπ‘™π‘’π‘‘π‘–π‘œπ‘› 20.0 Β π‘šπΏ Β  0.0200  𝐿 Β π‘ π‘œπ‘™π‘’π‘‘π‘–π‘œπ‘›1000 Β π‘šπΏπ‘šπ‘œπ‘™π‘’π‘  Β πΆπ‘–π‘‘π‘Ÿπ‘–π‘  𝐴𝑐𝑖𝑑0.0006358 Β π‘šπ‘œπ‘™π‘’π‘  Β πΆπ‘–π‘‘π‘Ÿπ‘–π‘  𝐴𝑐𝑖𝑑 𝐿 Β πΆπ‘–π‘‘π‘Ÿπ‘–π‘  𝐴𝑐𝑖𝑑 Β π‘ π‘œπ‘™π‘’π‘‘π‘–π‘œπ‘›0.0200  𝐿 Β π‘†π‘œπ‘™π‘’π‘‘π‘–π‘œπ‘› 0.0318  𝑀 Β πΆπ‘–π‘‘π‘Ÿπ‘–π‘ Β π΄π‘π‘–π‘‘π‘€π‘œπ‘™π‘Žπ‘Ÿπ‘–π‘‘π‘¦ Β π‘œπ‘“ Β πΆπ‘–π‘‘π‘Ÿπ‘–π‘  𝐴𝑐𝑖𝑑 Commonly missed steps:Titration curvesMismatching Β volume Β and Β concentration Β (look Β for Β β€œof” Β statement)A moreadvancedto analyze an acid-base reaction is to create a titration curve. A titrationUsingM1V1 M2wayV2 (this Β isn’t Β dilution)curve will have the volume of the titrant added from the buret as the independent variable and thepH of the solution as the acid and base mix as the dependent variable. The graph below is forthe addition of a base such as NaOH from a buret into a beaker containing an acid. As the titrant(the base in this example) is added, the pH will slowly rise and then undergo a large sudden risebefore leveling off again. As you can see in the graph below, the shape of the curve will beslightly different depending on whether the acid being titrated is a strong or a weak acid. You willgain a more detailed understanding of titration curves during the course of this experiment.The titration curve for theaddition of a base to astrong acid and for theaddition of a base to aweak acid. The area ofthe graph where the pH isnot significantly changing(between 0.0 mL and 20mL in this example) iscalled the buffer region.2Determination of the Ka of a Weak Acid and the Kb of a Weak Base from pH Measurements

Strong acids versus Weak Acids The strength of an acid is measured by its ability to donate a proton (H ); the strongest acidsdissociate 100% in water, donating all of their protons to water. For example, when HCl donates its proton in water, the proton bonds to a water molecule to form a hydronium ion (H3O ),HCl (aq) H2O (l) H3O (aq) Cl(aq) There are six strong acids: HCl, HBr, HI, HNO3, HClO4, and H2SO4. For any strong acid, [H3O ] [HA]initial because it dissociates 100%. Because of this, calculating the pH of the solution of astrong acid is relatively simple. The pH of a solution can be determined: All Acids: pH log [H3O ]Strong Acids only: pH log [HA]initialBeyond the six acids mentioned above, all other acids are classified as weak acids because theytypically dissociate less than 100% in water. For example, acetic acid, HC2H3O2, dissociatessomewhere around 10 %. Because it does not dissociate 100%, we can say that the dissociationof a weak acid is an equilibrium process.HC2H3O2 (aq) H2O (l) H3O (aq) C 2H 3O 2 (aq)Similarly for the weak acid benzoic acid, the reactionwould beHC7H5O2 (aq) H2O (l) H3O (aq) C 7H 5O 2 (aq)In general, the equation for the dissociation of the weakacid, HA is –HA (aq) H2O (l) H3O (aq) A (aq)Ka provides a quantitativemeasure of the degree towhich an acid dissociates. Asmall value for Ka ( 1)indicates that the acid remainsmostly as molecules (HA) insolution. On the other hand, alarge value for Ka ( 1)indicates that the acid hasdissociated to a greater extent-)(ASince the reaction of a weak acid with water is an equilibrium process, an equilibrium expressioncan be written -Ka [H3O ] [ A ][HA]The subscript β€œa” on K is used to denote that the reaction involves an acid and is called the aciddissociation constant.The Henderson-Hasselback equationUsing the Ka expression above we can derive a relationship that is very useful when working withtitration curves such as in this lab: -Ka [H3O ] [ A ][HA]First taking the logarithms of both sides of the above equation,log K a log[H3O ] log[A – ][HA]Determination of the Ka of a Weak Acid and the Kb of a Weak Base from pH Measurements3

then multiplying by –1 gives: log K a log[H3O ] log[A – ][HA] Substituting pKa for –log Ka and pH for –log [H3O ], (p just means –log)pK a pH log[A – ][HA]Finally re-ordering, yieldspH pK a log[A – ][HA](2)This equation is known as the Henderson-Hasselback equation. It relates the pH of solution tothe equilibrium constant (Ka) of the acid present in the solution, and is the basis of how we willdetermine the value of Ka for our unknown acid.Analyzing a titration curve- The Importance of the pH at the half-equivalence point.When a strong base (from a buret for example) is added to a weak acid (in a beaker under theburet for example), the strong base will provide hydroxide (OH ) ions. These ions will react withthe acid (HA) as in the equation below:-HA OH H2O A-Consider three points during the titration:-1. Before the you have added any base (OH ):Considering the moment just before the strongbase is added to the acid. All that is present inthe flask would be the acid HA.HA -OH H 2O A-Only HA in beakerOnly HAis present2. When the acid-base reaction is complete(equivalence point):On the other hand, considering when theequivalence point is reached. The acid (HA)and the base (OH ) will have destroyed eachother. All that would be present in the beakerwould be the product A .HA OH H 2O AHave destroyed each other4Only A- in beakerOnly ApresentDetermination of the Ka of a Weak Acid and the Kb of a Weak Base from pH Measurements

3. What about when we have added halfthe base need to get to the equivalencepoint?Half of the acid (HA) will remain, and half theacid will have been turned into A . (itsconjugate base) In other words, [HA] [A ].HA -OH H 2O Β½ HA still presentA-Β½ of HA has turned into A-Β½ will be HA, and Β½ will be A-Considering this point further and returning to the Henderson-Hasselback equation:pH pK a log[A – ][HA]-At half-way to the equivalence point, [HA] [A ]. Therefore, they would cancel[A – ]pH pK a log[HA]The equation would therefore simplify topH pKa log 1Since the log of 1 is 0,At half- way to the equivalence pointpH pKa (The pH of the solution is the pKa of the acid.Example: Determining the Ka of an acid from a titration curveA sample of 10.00 mL of dilute HNO2 solution was titrated with 0.1 M NaOH solution. Theequivalence point was reached after 10.10 mL. The half-titration point, therefore, was at5.05 mL. The pH that corresponded to that volume of titrant was 4.75, so the value of Kacan be calculated as follow ispH 4.75 athalf-pointThereforepHpKa 4.75-log Ka 4.75log Ka -4.75Ka 1.78 x 10-5Volume of NaOH addedDetermination of the Ka of a Weak Acid and the Kb of a Weak Base from pH Measurements5

Titration of a weak base with a strong acid.- Weak bases will form OH when they are placed in water. The do this by accepting a proton (H )from water. In this lab we will be working with the weak base ammonia (NH3). As a weak base,the reaction it undergoes in water is an equilibrium process where the equilibrium constant iscalled the base dissociation constant (Kb). The process for a weak base can be generallydescribed by the equations:B (aq) H2O (l) OH-(aq) BH - Kb [OH ] [ BH ][B](aq)More specifically for this lab:NH3 (aq) H2O (l) OH(aq) NH4 -(aq) Kb [OH ] [ NH4 ][NH3]You will be determining the value of Kb for NH3 as well as the molarity of an unknown NH3solution.Other Acid-Base ConceptionsIn this lab, you will also briefly study solutions which can resist changes when an acid or or baseis added to them. These solutions contain a weak acid and its conjugate base and are calledbuffers. In addition, you will briefly look at the effect salt can have on the pH of water through theprocess of hydrolysis.6Determination of the Ka of a Weak Acid and the Kb of a Weak Base from pH Measurements

ProcedureSafety: Wear goggles when working with acids and bases. The HCl, and NH3 solutions arerelatively dilute; however you should avoid contact and clean up spills immediately.Waste: All waste can go down the sinkPart A. Titration of unknown monoprotic acid using NaOH and an indicatorIn this part of the lab, you will titrate the unknown acid using the indicator phenolphthalein. Thiswill allow you to determine the molarity of the acid again and compare your results to thoseobtained with the pH sensor in Part C.MaterialsFor this part you will need the following: a 250 mL beaker, 50-mL buret, funnel, 20 mL pipetteNote: If you just performed part C, start with step 41. Obtain approximately 125 mL of the standardized NaOH from the carboy in the front of theroom. Record its molarity in your notebook.2. Clean a 50 mL buret with deionized water and then rinse the buret three times with thestandardized NaOH. Be sure to drain some the solution through the tip. This will get rid of anyair bubbles in the tip of the buret.3. Fill the buret to 0.00 mL mark with the NaOH using a funnel.4. Pipette 20.00 mL of your unknown acid solution to be titrated into a 250 mL beaker. Also add 3drops of phenolphthalein. Record the unknown number in your notebook.5. Add about 20 mL of water using a graduated cylinder.6. Record the initial buret reading of NaOH to the nearest 0.01 mL.7. Add NaOH from the buret into the sample of the unknown acid until the indicator changes fromclear to pink.8. Record the final buret reading of NaOH to the nearest 0.01 mL.9. Dispose of the sample down the sink.10. Repeat this titration one more time.11. Using the volume of NaOH, its given molarity, and the volume of acid use, calculate theMolarity of the unknown acid.Determination of the Ka of a Weak Acid and the Kb of a Weak Base from pH Measurements7

PART B. Computer Setup and pH Sensor CalibrationMaterialsTo calibrate the pH Sensor you will need the following: wash bottle, distilled water, three beakers(50-150 mL in size), buffer solutions of high pH (e.g. pH 7) and low pH (e.g. pH 4), pH Sensor.1. Turn on Science Workshop interface and the computer if they are not already on.2. Connect the plug of the pH Sensor to Analog Channel Aon the interface.3. On the desktop, open the program Data Studio, thenselect Open Activity, and then open the file weakacid.sws inthe Chem 1B folder on the desktop. The file will display agraph display of the pH versus time and a data table of pHand mL added.ScienceWorksho500P Inp DATALOGGIN GIN STRUCTIONSAscieSC terfacentifi OcpH sensorAPNAAscieSLOCntifi GOCHcANNELSSWo cienrksh ceop 300InterfacepH sensoramplifierRemovethe bottleof buffersolution.4. Put distilled water into the wash bottle and into one of the beakers. Put about 50 mL of the pH4 buffer solution in one of the other two beakers and about 50 mL of the pH 10 buffer solution intothe third beaker.5. Remove the pH electrode from its storage bottle of buffer solution.6. Use the wash bottle to rinse the end of the electrode. If the pH electrode has not be soakingwhile stored, soak the electrode in a beaker of distilled water for 10 minutes.7. In the Experiment Setup window, click Calibrate Sensors at the top.8. Place the pH sensor in the pH 4 buffer.9. Find were it says β€œCalibration point 1” and in the β€œStandard Value Box” check that the value isset to 4.000. If is does not say 4.000, change it to 4.000.10. When the