Experiment 9: DETERMINATION OF WEAK ACID IONIZATION CONSTANT & PROPERTIES OF A BUFFERED SOLUTION

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Peregrine Caldwell
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1 Experiment 9: DETERMINATION OF WEAK ACID IONIZATION CONSTANT & PROPERTIES OF A BUFFERED SOLUTION Purpose: Prt I: The cid ioniztion constnt of wek cid is to be determined, nd the cid is identified ccordingly. Prt II: The effect of dding cid nd bse to buffered solution is to be exmined. Introduction: The ph of solution is defined by the following eqution: ph = log[h + ] Eqution 1 or ph = log[h 3 O + ] The ph of solution tells us the cidity (or bsicity) of the solution. It is convenient wy of expressing the concentrtion of the hydrogen ion, H +, or hydronium ion, H 3 O +, in queous solutions. For exmple, insted of sying tht solution contins M H 3 O +, we cn simply sy tht it hs ph of 4. One cn mesure the ph of solution experimentlly in two wys. The first method utilizes chemicls clled INDICATORS, which re sensitive to ph. These substnces hve colors tht chnge over reltively short ph rnge (bout 2 ph units) nd cn, when chosen properly, be used to roughly determine the ph of solution. Two very common indictors re litmus, usully used on pper, nd phenolphthlein, the most common indictor in cidbse titrtions. The other method for finding ph is with ph meter. In this device, two electrodes (or single combintion electrode) re immersed in solution. One electrode is sensitive to [H + ] nd the potentil between the two electrodes is relted to the ph. The ph meter is designed so tht the scle will directly furnish the ph of the solution. It gives much more precise mesurement of ph thn does typicl indictor, nd is ordinrily used when n ccurte determintion of ph is needed. Some cids nd bses undergo substntil ioniztion in wter, nd referred to s being strong becuse of their essentilly complete ioniztion in resonbly dilute solutions. Other cids nd bses, becuse of incomplete ioniztion (often bout 1% ionized in 0.1 M solution), re clled wek. Hydrochloric cid, HCl, nd sodium hydroxide, NOH, re typicl exmples of strong cid nd strong bse respectively. Acetic cid, HC 2 H 3 O 2, sometimes bbrevited HOAc, nd queous mmoni, NH 3 (q), re clssic exmples of wek cid nd wek bse. A wek cid will ionize ccording to the Lw of Chemicl Equilibrium: HA (q) H + (q) + A (q) Eqution 2 or HA (q) + H 2 O (l) H 3 O + (q) + A (q) 87

2 88 EXPERIMENT 9: WEAK ACID K & PROPERTIES OF BUFFER SOLUTION At equilibrium, or K = K = + [H ][A ] [HA] + [H3O ][A ] [HA] Eqution 3 K is referred to s the cid ioniztion constnt, nd is chrcteristic of the cid. A similr reltion cn be written for the solutions of wek bse. DETERMINATION OF K OF A WEAK ACID The vlue of K for wek cid cn be found experimentlly in severl wys. Perhps the most obvious wy is to simply mesure the ph of solution of the cid of known molrity by setting up n ICE tble. The [H + ] t equilibrium cn be clculted from the ph, using Eqution 1. According to the stoichiometry shown in Eqution 2, we see tht [H + ] must equl to [A ], since there is no other source of H + but from HA. To obtin the [HA] t equilibrium in Eqution 3 we subtrct [H + ] from the initil [HA] to correct for the mount of HA lost by the dissocition. By substituting [H + ], [A ], nd [HA] equilibrium vlues into Eqution 3, K cn be clculted. HA H + + A I initil [HA] 0 0 C x +x +x E initil [HA] x x x where x = [H + ] which cn be determined from ph mesurements Another very simple procedure, which involves very little clcultion, is ccurte nd does not even require the knowledge of the molrity of the cid. A smple of wek cid, often solid, is dissolved in wter. The solution is divided into two equl prts by volume (Soln 1 nd Soln 2), ech contining the sme molr concentrtion of HA. With the sme volume of solution involved, ech hs the sme number of moles of HA s well. Soln 1 is titrted to phenolphthlein end point with NOH, nd ll of its HA is converted to A by the rection: OH (q) + HA(q) A (q) + H 2 O (l) Eqution 4 The number of moles of A produced equls the number of moles HA tht ws in Soln 1 becuse its stoichiometric rtio is 1:1. It is lso equl to the number of moles HA in Soln 2. The two solutions re then mixed, nd the ph of the resultnt solution is recorded. In this resultnt solution becuse [HA] equls [A ], these two terms in Eqution 3 cncel out, leving us with simply [H + ] = K.

3 EXPERIMENT 9: WEAK ACID K & PROPERTIES OF BUFFER SOLUTION 89 K = + [H ][A ] [HA] K = [H ] Eqution 5 By finding the log of both sides of the eqution, nd then multiplying both sides by 1, we see tht pk = ph. log K = log [H + ] pk = ph The resultnt solution described bove is sometimes referred to s being hlfneutrlized. Remember tht it is prepred by mixing equl volumes of solution tht hs been neutrlized nd solution tht hs not. This gives us solution where [HA] = [A ] nd pk = ph. By mesuring the ph of such solution we would be ble to determine its pk nd clculte its K. HOW BUFFERS WORK Buffers re solutions tht re remrkbly resistnt to ph chnges cused by ddition of n cid or bse. Buffers contin pproximtely equl mount of wek cid nd the slt of its conjugte bse (or wek bse nd the slt of its conjugte cid). In this experiment we will be deling with the former combintion (wek cid/conjugte bse). The solution used in the forementioned procedure for finding K of wek cid is n exmple of buffer solution. The solution contined equl mounts of the wek cid (HA) nd the nion (A present s its slt. If smll mount of strong cid is dded to the solution, the H + ion would rect with the A ion present, keeping the [H + ] bout where it ws before the ddition. Similrly smll mount of strong bse dded to the solution would rect with the HA present, producing some A ion nd wter but not pprecibly chnging the [OH ]. In contrst, if similr mounts of cid or bse were dded to n unbuffered smple (such s plin wter) the ph chnge could esily be severl ph units. HANDLING SIGNIFICANT FIGURES IN ph [H + ] CONVERSIONS The number of deciml plces in the ph determines the number of significnt figures in the corresponding [H + ]. e.g. ph = 2.35 hs two deciml plces, so [H + ] = 4.5x10 3 M (2 sig.fig.) ph = 8 hs no deciml plces, so [H + ] = 10 8 M (no sig. fig.) The number of significnt figures in [H + ] determines the number of deciml plces in the corresponding ph. e.g. [H + ] = M hs 3 significnt figures, so ph = (3 deciml plces) [H + ] = 1x10 2 M hs one significnt figure, so ph = 2.0 (1 deciml plce) Sfety Precutions: Wer your goggles t ll times. Avoid contct with the queous solutions of NOH nd HCl. Both chemicls re corrosive, cn cuse burns nd irrittion to your eyes nd lungs.

4 90 EXPERIMENT 9: WEAK ACID K & PROPERTIES OF BUFFER SOLUTION Procedure: Work with one prtner Prt I: Determintion of the Dissocition Constnt of Wek Acid, K. 1. Obtin n unknown solid cid smple from your instructor. RECORD the unknown number in your lb notebook. 2. Rinse buret twice with smll portions (~5 ml) of the 1 M NOH solution. Remember to get rid of the ir bubble t the tip of the buret. Fill the buret to the ml mrk with 1 M NOH. 3. Using 50mL grduted cylinder mesure out 50.0 ml of deionized wter nd pour it into clen nd dry 250mL Erlenmeyer flsk. Dissolve bout hlf of your unknown solid cid smple in the wter nd stir thoroughly with glss stirring rod. 4. After you re sure ll of the unknown solid cid hs dissolved, pour the entire solution into the 50mL grduted cylinder. Trnsfer exctly hlf of this solution bck into the 250mL Erlenmeyer flsk. Gently slide mgnetic stir br into the flsk. 5. Add 23 drops of phenolphthlein indictor to the solution in the flsk nd titrte the solution to fint pink endpoint using the 1 M NOH in the buret. (Volume redings do not hve to be tken here). Add the hydroxide solution slowly while the content of the flsk is being stirred. As the end point pproches, dd the solution drop by drop until the solution hs permnent, light pink color tht persists for 30 seconds. You cn split drops by llowing the drop of NOH solution to build on the buret tip nd then wshing it into the flsk with deionized wter. 6. Add the remining prt of the solution from the 100mL grduted cylinder to the flsk contining the neutrlized cid nd stir to mix. 7. Follow directions in Appendix 5 on the use of the ph meter. Clibrte the ph meter with the ph 4 Buffer Solution nd then record the ph of the hlfneutrlized solution from Step For the first tril (only), discrd the solution nd repet the determintion strting t Step 3 with the other hlf of your solid wek cid. For the second tril, do NOT discrd the hlfneutrlized solution. It will be used s buffer in Prt II. CALCULATIONS: Write netly on your own pper. Do individully. DO NOT COPY FROM YOUR PARTNER. For ech tril, clculte the [H + ] from the observed ph nd stte the K of your unknown cid. Clculte the verge K from the two trils. Below re the literture vlues for the equilibrium constnt of possible cids. Bsed on your results, which is your unknown cid? Acid d,lmndelic cid chlorocetic cid sodium bisulfite sodium bisulfte K 4.3 x x x x 10 2

5 EXPERIMENT 9: WEAK ACID K & PROPERTIES OF BUFFER SOLUTION 91 Prt II. Properties of Buffered Solution. In this prt of the experiment, note tht you will be using 0.1 M nd not 1 M NOH. 9. To 25 ml of the second buffer (hlfneutrlized) solution from Prt I (Step 8) dd 10 drops of 0.1 M NOH nd mix thoroughly. Record the ph of the solution in your lb notebook. 10. To nother 25 ml of your second buffer solution from Prt I (Step 8) dd 10 drops of 0.1 M HCl, mix thoroughly, nd record the ph of the mixed solution in your lb notebook. 11. Mesure the ph of 50 ml of tp wter. If the ph mesured is not between 5.0 to 8.0, discuss your result with the instructor before continuing with the experiment. 12. To 25 ml smple of the tp wter dd 10 drops of 0.1 M NOH. After mixing, record the ph. 13. To nother 25 ml smple of tp wter, dd 10 drops of 0.1 M HCl. After mixing, record the ph. 14. CLEANUP: The solutions my be discrded in the sink becuse they re just wek cid, wek bse or slt solutions. CALCULATIONS: Write netly on your own pper. DO NOT COPY FROM PARTNER. Clculte the chnges in ph ( ph) for the buffer due to () dding the queous NOH, nd (b) dding the queous HCl. Show your clcultions. (ph of the buffer ws mesured in Prt I.) h ph = new ph originl ph Clculte the ph for tp wter due to () dding the queous NOH, nd (b) dding the queous HCl. Show your clcultions. PreLbortory Assignment: 1. Prepre your lb notebook. This experiment is simple enough tht you should be ble to design your own tbles to record dt. As usul, your notebook will be checked t the beginning of the lb. COME PREPARED! 2. A solution of NCN hs ph of 9.5. Find the [H + ] in the solution. Is the solution cidic or bsic? 3. A M solution of the wek cid HB hs ph of Wht re the [H + ], [B ], nd K for this cid? 4. A solid cid is dissolved in wter. Hlf the solution is titrted to phenolphthlein end point with NOH solution. The neutrlized nd cid solutions re then mixed nd the ph of the resulting solution is found to be Find K of the solid cid. 5. Assuming your buffered solution contins cetic cid, HOAc, nd the sodium slt of its conjugte bse, NOAc. Show the net ionic equtions tht occur when this buffered solution rects with: () dded strong cid, H +, or (b) dded strong bse, OH. PostLb Questions: 1. In Prt I, you were to use hlf of the solid unknown wek cid for ech tril. Does it mtter if you did not use EXACTLY hlf nd therefore hd different mounts of the solid cid for ech tril? Would it increse, decrese or hve no effect on the cid ioniztion constnt you re reporting? Explin. (continued next pge)

6 92 EXPERIMENT 9: WEAK ACID K & PROPERTIES OF BUFFER SOLUTION 2. In Prt I, why must the unknown cid be wek cid? 3. Biochemists nd biologists often use the HendersonHsselblch eqution in working with buffers: [A ] ph = pk + log [HA] HA + NOH H 2 O + N + A When n cid solution is hlfneutrlized, hlf of the HA hs been converted to A, nd hlf of it is still in the form of HA. Therefore [HA] = [A ]. Wht hppens to the HendersonHsselblch eqution when [HA] equls [A ]? How is it simplified? 4. In Prt II, wht rection is tking plce when queous NOH is dded to buffer so tht the ph does not increse? Wht rection is tking plce when queous HCl is dded to buffer so tht the ph does not decrese? Answer in full sentences nd lso write net ionic equtions to illustrte your nswers. You cn use the generic formul HA nd A s the cid nd its conjugte bse in the buffer. 5. The purpose of Prt II ws to show how ddition of strong cid nd of strong bse does not significntly ffect the ph of buffer. Wht role does the tp wter ply? Explin fully. Specil Instructions for the Forml Lb Report: 25% of your report will be bsed on how well you follow the formt described in Appendix 8 nd 9. 25% will be bsed on how well you design your tbles nd develop the prgrphs of your introduction, discussion nd conclusion. 25% will be bsed on how well you nswer the prelb nd postlb questions. Review Appendices 7 nd 8 on wht should be in the forml lb report. There re no Clcultions & Results Pges provided for this experiment. You must design your own. Present ll dt nd results in tble formt. Refer to previous experiments s guideline s to how tbles re set up. Write seprte discussions, error nlyses nd conclusions for Prt I nd II. In Prt I, be sure to stte your unknown number nd the identity of your unknown cid in your discussion nd conclusion. Discuss whether your results re relible. You cnnot clculte stndrd devition for only two trils, but you cn clculte the percent difference between the two results. Why would one be interested in the percent difference? In Prt II, first explin wht buffer is nd then discuss how the clculted illustrte the effectiveness of your buffer. ph vlues you hve

Experiment 9: WEAK ACID IONIZATION CONSTANT & PROPERTIES OF A BUFFERED SOLUTION

Experiment 9: WEAK ACID IONIZATION CONSTANT & PROPERTIES OF A BUFFERED SOLUTION Purpose: Prt I: The cid ioniztion constnt of wek cid is to be determined, nd the cid is identified ccordingly. Prt II: The