Hart %
Half-Titration of a Weak Acid Emily Hart Kailash Raman Analise Nicholson Mr. Lannen Febrary !" #$%&
Hart # Abstract' The main reason for this e()eriment *as to be )ro+e that sin, the method of halftitration one cold determine the K a and the )K a of a *eak acid" in this case acetic acid. ome key reslts *old be the calclated K a of acetic acid is %.# (%$-& and the calclated )K a is /.!#" *hich both are +ery close to the acce)ted +ales. A fe* ma0or )oints of discssion and main conclsions *old be that the e()eriment e() eriment *as sccessfl" the hy)othesis *as acce)ted and kno*in, the )H of certain thin,s is actally ac tally +ery hel)fl in the real *orld. 1ntrodction' The )r)ose of the e()eriment *as to determine the K a of acetic acid by findin, the )H of the half-titrate soltion. The hy)othesis *as that if the concentrations of the sodim h ydro(ide and the acetic acid is kno*n" as *ell as the )H" one can find the )K a and in trn the K a. Make sre to take cation *hen sin, the sodim hydro(ide" as it is castic and a+oid s)illin, it on clothes or skin" as *ell as acetic acid" acetic acid cases brns *hen it comes into contact *ith the skin. Also *ear ,o,,les at all times drin, labs and to dis)ose of all ch emicals as told by the instrctor. Materials' • • • • • • • • • • • •
Lab2est Lab2est A)) 3ernier )H ensor &$ mL bret 4ret clam) 5ne rin, stand %.$ M sodim hydro(ide" Na5H" soltion %.$ M acetic acid" H6#H75#" soltion 8henol)hthalein indicator soltion 9istilled *ater 8lastic 4eral )i)ets :tility clam)
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8rocedre' %. Find and )t on ,o,,les. #. :sin, a bret clam) connect a &$ mL bret onto a rin, stand. First rinse then fill fill the bret *ith the %.$$ M acetic acid soltion. HANDLE ACETIC ACID WITH CARE. 7. :sin, the bret transfer )recisely #&.o mL of the acetic acid into the #&$ mL beaker. /. :sin, a )lastic )lastic micro-)i)et to to take a small small amont of the acetic acid from from the #&$ mL beaker. 9ra* eno,h to fill the )lastic )lastic blb abot ; if the *ay fll" set it aside becase it *ill be sed later. &. Add abot # dro)s of the )henol)hthalein into into the beaker of acetic acetic acid. <. 6onnect the )H ensor to the Lab2est" a ne* file shold be o)en" if not select Ne* from the File men. =. >et a))ro(imately &$ mL of %.$$ M Na5H soltion. HANDLE SODIUM HYDROXIDE WITH CARE. ?. 4e,in the half-titration. a. :sin, the the rin, stand stand clam) clam) to hold the the )H ensor in in )lace" )ositi )osition on it so that that the ti) ti) of
the )robe is sbmer,ed in the beaker com)letely. b. tir the acidic soltion careflly and ,ently. ,ently. c. 9o not se the data collection@ monitor the )H of the reaction sin, the Lab2est. d. 5btain a ne* )i)et to slo*ly add the %.$$ M Na5H soltion" a))ro(imately a))ro(imately & dro)s at a time" to the beaker *ith acetic acid soltion. !. 9o the titration titration careflly careflly.. When When the reaction reaction a))roached a))roached the ei+alence ei+alence )oint" )oint" arond arond )H <" contine to add the Na5h soltion dro) by dro). When the ei+alence )oint is reached" the )H *ill increase +ery +er y fast and the indicator *ill chan,e to a li,ht )ink color. 1f needed" add another dro) of the base so that the reaction is 0st )ast the ei+alence )oint. The )H *ill not increase ra)idly beyond the ei+alence )oint" *hich is arond )H %$. %$. :sin, the the )i)et from te) te) /" add all the acetic acetic acid from it" it" into the beaker beaker of the reaction reaction mi(tre. The mi(tre shold be sli,htly acidic once a,a in@ the )H readin,s and the indicator color shold sho* this.
Hart / %%. Add Na5H a,ain a,ain dro) by dro)" dro)" to the the beaker of the the reaction reaction mi(tre" mi(tre" ntil ntil the ei+alence )oint is reached as )recisely as )ossible. A +ery li,ht )ink color from the indicator is +isible. This is the half-titrated soltion" becase it *as netraliBed )recisely of #&.$ mL of the ori,inal &$.$ mL of acetic acid that *as measred ot into the bret. %#. 8t the remainin, remainin, #&.$ #&.$ mL of acetic acetic acid from the the bret to the #&$ mL beaker of reactio reaction n mi(tre. tir the mi(tre. Read and record the )H. %7. When the testin, testin, is com)leted com)leted"" dis)ose of the the reaction reaction mi(tre. mi(tre. Rinse the the )H ensor *ith *ith distilled *ater. 9ata' Titration Results Ei+alence )oint )H )H of half-titrated soltion
Trial 1 ?.7? /.!#
Analysis' 1. Calc Calcul ulat atee te te pK a an! K a usin" te results o# $our testin". )HC )K a D lo,6#H75#-GH 6#H75#GI /.!#C )K a D lo,%I %.&'( )* aC -lo,K aI %$-/.!#C%$lo,K aI K aC %$-/.!# * a( 1.' +1,-/
9iscssion' The e()eriment *orks" determinin, the )K a and the K a *as a sccess and they match ) almost e(act com)ared to the acce)ted acc e)ted +ale. The data *as collected and an d all the analysis )roblems cold be sol+ed *ith no )roblems. The )r)ose *as to be able to find the )K a and the K a sin, the method of half-titration" *hich is *hat *as done drin, the e()eriment and sed to sol+e for those as stated before. o in trn the )r)ose *as flfilled.
Hart & The calclations sho* that acetic acid is a *eak acid becase of ho* small the K a is. They also sho* that the e()eriment and a nd )r)ose *as ltimately flfilled. The main theory behind this lab *as that instead of findin, the ei+alence )oint sin, titration" also sin, the method of half-titration cold also be sccessfl in doin, so. Then" in trn" the )K a cold be fond and then after that sin, )re+ios kno*led,e of different eations sch as" )HC -lo,HDG and )5HC -lo,5H-G" the K a cold also be determined. 1n the real *orld many )eo)le need to monitor the )H balances of many thin,s" like )ools" drinkin, *ater and e+en their blood. o it is im)ortant to realiBe e+en the sli,htest flctation of that eilibrim in their body and their en +ironment *ill ha+e detrimental affects. For e(am)le" acid rain can kill of )o)lations of fish casin, Jdead lakes as *ell as if someones blood becomes too acidic or basic they can die. 6onclsion' 1n conclsion the calclated )K a and the K a are +ery close to the acce)ted +ales" *hich means the hy)othesis *as acce)ted. Which means that the e()eriment *as sccessfl and the lab *ent smoothly. 2estions' 1. 0in! 0in! te te acce)t acce)te! e! alue aluess #or te te pK a an! K a o# acetic aci!. Ho2 2ell !o te acce)te! alues co3)are 2it $our calculate! alues4 E+)lain. The acce)ted +ales for the )K a and K a of acetic acid are /.=& and %.=< (%$-&" in that
order. These These +ales com)ared *ith the calclated +ales are +ery ,ood" only a fe* tenths of a *ay off. '. E+)lain E+)lain 2$ 2$ te )H at te te al#titr al#titratio ation n )oint )oint is e5ual to te te pKa in $our e+)eri3ent.
Hart < The half-titration )oint is eal to the )K a in the e()eriment becase both concentrations of the soltions *ere %.$$ and lo,%I is eal to Bero" so then in this case the formla' )HC )K a D lo,6#H75#-GH 6#H75#GI" *old be able to be taken do*n to' )HC )K a. 6. E+)lain o2 tis test coul! coul! 7e !one !one usin" usin" onl$ an in!icator in!icator solution an! no electronic 3eans o# 3easurin" )H. :sin, only an indicator soltion *old still *ork becase" like )henol)hthalein" most
indicator soltions only chan,e colors *hen the soltion reaches at a certain )H" and in the case of )henol)hthalein" it chan,es to a li,ht )ink at arond ? and %$ on the )H scale 5)hardtI. o the electronic means of measrin, the )H is basically not needed" bt is hel)fl to kno* *hen eilibrim is a))roachin, and to hel) the e()erimenter kno* *hen to start slo*ly do*n on addin, the base. Works 6ited 5)hardt" 6harles E. Acid-4ase 1ndicators. Acid-Base Indicators Indicators. Elmhrst 6olle,e" n.d. Web. %% Feb. #$%&.
