Chemistry 271. section 22xx Exam I. 10/6/10 2r/L. (20 pts: 2 points each) True or false: Place an X in the appropriate columnStatementTrue FalseThe pH at the half-equivalence point is always the average of the pK" at thebeginning of a titration and at the equivalence point.xThe histidine side chain is neutral at pH 4.XAt equilibrium the rates of the forward and reverse reactions are both zero.XThe Henderson-Hasselbach equation is not correct when [A-l is too large.XWhen we add equations we add equilibrium constants.XThe buffering range for a weak acid is between the pK" and pK"+ 2, but notbelow the pK".xKo and K, are equal to each other if there are no gases involved in theequilibrium.XThe percent dissociation of a weak acid decreases as its concentrationdecreases.xIn multiple equilibrium problems, all equilibria must be satisfied.XThe equilibrium constant of a reverse reaction is the reciprocal of theequilibrium constant of the corresponding forward reaction.XScore for the pageChemistrv 271. section22xx Exam l.1016ll0 3n6) l^/t{^ :ft I /(q/ a o^ q*'l,t*tv-, }u yt\,L.f,,"', sh,Fb L\-/ veleva- lu glm$f^ /^,wryW-4,4&wL,"u.h^pJc-r177zV t,Z\l9 A nF/n6</^r*K % u4[ln{'-' I <fhL l,7l'?tL(b; 3 pts) Write the K, expression for the following reaction:BaSOo (s) + Ba2* (aq) +Jlw'1/. ,.i ..1*2 ( lo o r* IDO/ : l.? * lO^ /l{ nsl Does the amount of dissolved barium ion jepind on the amount of solid barium sulfate sitting at( +L )ne bottom of the beaker? Circle one: YES ( NO)\_7 \-/(d; 6 pts) It turns out that the numerical value of K" is 1.1 x 10-10. The small value for K" for some sulfatesalts is the basis of selective precipitations in quantitative analysis, in which, for example, one can use astandardized sodium sulfate solution to measure the barium ion concentration in a test sample.Calculate the concentration of barium ion in solution if the [SO.]l is 75 mM. If the initial barium ionconcentration before precipitation with NaSOo was 125 mM, what percentage of the barium ionremainsin solution? | //, | | -(o / ^r l.+/^)? -,-," -x1/--'L o^nJI@ (or l'rx to-1 h)t-/t..t? rto1l ,,qr'* tD,'' -rzen^rylzryc zo(LsM Ao2. (24 nts) Chemical Equilibria(a;9 pts) State LeChatelier's principle. Define the reaction quotient Q, and give the equivalent ofLeChatelier's principle in the language of thermodynamics.2il" S(*f.u =+T\'I/[-, - r. l', fo \'^1- ,^,,'1'uJ fto,(.2tJ.-.?L/r+ x lo IqtzE MScore for the pageChemistrv 271. section 22xx Exam I. 10/6/10 4|/(e; 4 pts) Why do we use NasO;instead of sulfuric acid for the barium precipitation (other than safety)?6Df sor'-J (n su(( ^Yo.""^ 3 b' - , s-+1'-- /yJo('.*t, L{ - }}'tct r3. (18 pts) Biological Applications(a;6 pts) Draw the structure of the Lysine side chain at pH7 and give its approximate pK".CoFWcl vZot t(pl& = lo,{' @ t- wrro'g o tt(b; 12 pts) Explain why measuring the pH dependence of an enzymatic reaction is a useful probe intomechanism. {ain whl, however, the experiment offers only a guide to identifying the active siteresidue typeQyls a concrete example, explain how a pH curve might suggest but not prove that aHIH^# - C*t/t( V"rt'vhlcltr(t-r^Oy{Lq t ttQ@r\ lrcsff * { R^r,z aCe< )unV-{ fw.. ety,bo a'cdtV h^tf e lQ {'{*r(t,r-<rr,?ch:hs/ tua\dar h-1" .pntbtSnr. Lt a,44^,'r'y'tcrlrr-hrr"ufc6DCt-/tT cv'L ba<. 'll^ Cur"rwe f" lq1. ({ ,n}ltt vqsw tl- . gu* { + QuL(,rtl> h q htrhuf,r* ,"r.h^ a.'- a(k'*lYz?fvat c*k; t ,-531^^tQVIr^lt-* Score for the page'y.ltots h bL f*l,,tl.(,S.tq B *82,/aut qr fa,/ (^^t =.1L( rflY\Lt{Chemistrv 271. section22xx Exam l.1016110 5n@f**twt4. (38 pts) Acid-base chemistry and multiple equilibriaConsider the diprotic acid malonic acid, with pKo's 2.83 and 5.89.(Malonic acid is an inhibitor of the important metabolic enzymesuccinate dehydrogenase.)(a; 8 pts) The first pKa is very low relative to a typical carboxylic acidlike acetic acid, PKo= 4.75. This means that malonic acid is a (circle one)T--tt*,lrnc [(<o+"r:,[Tr0, \Vl'lr-^l,v.l .'-\<.\@d,tillArTcrto+")cr?cf cl-2'8) = l,r0rlo-9 1 @Ase fw,?(L*fu={'.(8t'ro'3O@@pt4= -tV G) = l,g 3,o--l1-rrAo,.,r a weakerthe other hand, the second pK" of malonic acid is higher than the pK, of acetic acid. Explain why.9460 o go o? ,oI i f f" ' I I ll o*f"r*'Lnr,,, /$o rn"JV- oH+ ra--SAon o.cqr/.Iu-|-,!v40 o,tT tl it/o 5.Cq{ e\-OO3+on21 rrr oOloA--\og t" *-g 1<-ua12 f hf h}.i-o/-_Ao @*4 @+kzFt^ - ,lfk{,'Ir "1 S,.lsli h^ul s hL{l,WO c(*t^Y aljaut ula,r-,./- (tt ^"vrl, d k-. 4rAtul' pr-h^ 3 .W lt'rQ. br'<o,*, t,./< an 9*$;a pts) Why can we ignore the second pK,when calculating the pH of a malonic acid solution?- T1* o(;rsoor--f,'^ 6 tu $-rf fb"-tt^ s4rt*ts t*rlytr.kJ o( ?t*."h> fo!: Ik pYl '-twLl'.? &,^w fsF ,\,'2^1-;1,. .l(c; 6 pts) Calculate the pH of 0.150 M malonic acid, assuming (dubiously) that"x" << 0.150 M.l4rw *O,,l{O_T0=FO,l5e -Thrtn- +vr Yq,>o11C-+"r0-150'T+T/ = l.\{xlo-Lger aci4t f,rr^ttt 4m rh,,,.Score for the pageChemistry 271. section 22xx Exam I- 10/6/10 6f7(d; 9 pts) Sketch a curve of the pH as we titrate 0.150 M malonic acid with NaOH, ignoring dilution.Specify the pH for the beginning (the number youjust calculated in c), the two half-equivalence points,and the two equivalence poinis (the second one is at pH = 9.5_3). ,@6, b2st'1*k**l?(ff^*))d- r"-,1.'Arc.h{IoqI+6fI37t/K,z.t3C urr&q, Q. <tl^ #J ) rL. #'"P I^ Jt{.3tn@?kt-@5t&pAtr9"L(I(o< IF s,h-c*, ir x-f L((r))r'6@ G2,'*JaYY'un^<t4 bu{+-v ff,^tlYuyw+,aScore for the Dase IChemistry 271- section 22xx Exam I- 10/6/10 7|/(e; 4 pts) We did not learn to calculate the exact pH for diprotic acid titrations, but we do know how to setup the problem. For any point on a titration of malonic acid IlM being converted to HM- and M2- withadded NaOH, we know the [Na*] and we would like to calculaG the concentrations of five otherchemical species. one is hydroxide. write down the other four. (Not a trick question)lnrJ, Lurfi, [r,r r^-J, [nz-J @r* (;'^-"\(f; 7 pts) We need five equations to solve the problem. Two of them are the two K, relationships for the firstand second acid dissociation equilibria. Write down the last equilibrium relationship needed and thetwo conservation equations that we need to have fiv,e_equations. Hint: the total charge ofthe solution is6 l(,s= Lu',rJt urJ 6 /'Q) i{ k" - t+H6 Co : Lur^)+ Lr. y^JtT +u trt n cluqst+ +U LJ {, n,.br^at( 1"b^ 4 (rrU.tn ,\^4r t< 4PaeeScore21203t204t225/186t97lrlTotal/100Score for the