Identifying an Unknown Weak Acids Experiment
Transcript of Identifying an Unknown Weak Acids Experiment
ABSTRACT / SUMMARY
An exper iment i s conduc ted to iden t i f y an unknown
monopro t i c ac id by ob ta in ing the pKa va lue fo rm t i t ra t ion curves . 10
mi l l i l i t res o f the unknown ac id i s t i t ra ted w i th approx imate ly 0 .1 M o f
sod ium hydrox ide so lu t ion . Three t r ia l s a re conduc ted and there fo re
th ree t i t ra t ion curves a re p lo t ted . The va lues o f pKa and Ka o f the
unknown ac id f rom the t i t ra t ion p rocess a re 4 .537 and 2 .90 x 10 - 5
respec t i ve ly . Whereas , the pKa and Ka va lues ob ta ined f rom us ing
the in i t i a l pH o f the unknown ac id a re 4 .81 and 1 .53 x 10 - 5
respec t i ve ly . Theore t i ca l l y , the pKa and Ka va lues o f the unknown
monopro t i c ac id , wh ich i s be l ieved to be o f ace t i c ac id a re 4 .75 and
1 .76 x 10 - 5 . There fo re , an e r ro r o f 4 .48 percen ts o f the pKa va lue
and 64 .77 percen ts o f the Ka va lue f rom the t i t ra t ion p rocess a re
ca lcu la ted . Meanwh i le , an e r ro r o f 1 .26% and 13 .1% o f pKa and Ka
va lues ob ta ined f rom us ing the in i t i a l pH o f the unknown ac id i s
ca lcu la ted . There fo re , the re i s on ly a s l igh t d i f fe rence f rom the
theore t i ca l va lue compared to the va lues f rom th e t i t ra t ion p rocess .
Hence , the unknown monopro t i c ac id i s iden t i f i ed as ace t i c ac id and
the va lue de te rmined f rom us ing the in i t i a l pH va lue o f the ac id i s a
more accura te method . The exper iment i s comple ted and
success fu l l y conduc ted .
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INTRODUCTION
Ac id -base t i t ra t ion i s a method o f neu t ra l i za t ion p rocess wh ich
p rov ides in fo rmat ion regard ing the p roper t ies as we l l as na tu re o f
e i the r ac id o r base app l ied dur ing the p rocess . L ikew ise , i t i s use fu l
to de te rmine the mo lecu la r mass and pKa va lues o f the subs tances .
The end-po in t o f such t i t ra t ion can be mon i to red by us ing ind ica to rs
o r as what i s used in th is exper iment , a pH meter w i th e lec t rodes .
A t i t ra t ion curve i s a g raph o f measured pH va lues ob ta ined
f rom pH meter read ings versus vo lume o f t i t ran t be ing added in
m i l l ime t res . Bas ica l l y , the mos t impor tan t in fo rmat ion tha t i s
supposed to be ob ta ined f rom ac id -base t i t ra t ion i s the equ iva lence
po in t . The equ iva lence po in t i s theore t i ca l l y reached when the
number o f mo les o f base be ing added in to the Er lenmeyer f l ask i s
equa l to the number o f mo les o f ac id . I t occurs in the t i t ra t ion curve
in the reg ion where there i s a la rge and no t i ceab le change in pH w i th
a re la t i ve ly sma l l change in vo lume o f t i t ran t .
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AIMS / OBJECTIVES
The exper iment i s conduc ted to ach ieve ma in ob jec t i ve , wh ich
i s to iden t i f y the unknown monopro t i c ac id by compar ing the
ca lcu la ted average Ka va lues f rom the da ta ob ta ined f rom th is
exper iment w i th the Ka va lues o f some common ac ids found in a
genera l chemis t ry book . In o rder to ge t the Ka va lues fo r the
unknown ac id , i t can be ob ta ined by per fo rming t i t ra t ion p rocess
based on the pH va lue . Bes ides tha t , the Ka va lue can a lso be
de te rmined by us ing the in i t i a l pH o f the unknown monopro t i c ac id .
THEORY
I n th is exper iment , we w i l l be dea l ing w i th monopro t i c ac id .
Based on Brøns ted and Lowry , an ac id i s a p ro ton donor whereas a
base i s a p ro ton accep to r . Th is por t rays a very impor tan t idea to
unders tand ing monopro t i c and po lypro t i c ac ids and bases s ince
monopro t i c , as a mat te r o f fac t , i s bas ica l l y re fe r red to the t rans fe r
o f one p ro ton . On the con t ra ry , po lyp ro t i c co r responds to the
t rans fe r o f more than one p ro ton . There fo re , monopro t i c ac id i s an
ac id tha t can dona te one p ro ton wh i le po lypro t i c ac id i s an ac id tha t
can dona te more than one p ro ton . To be more p rec ise , monopro t i c
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ac id can re lease one p ro ton per mo lecu le and hence , have on ly one
equ iva lence po in t .
When a weak monopro t i c ac id (HA) i s d isso lved in wa te r , on ly
ce r ta in o f i t s mo lecu les w i l l d i ssoc ia te to y ie ld hydron ium ions ,
H3O + , and A- ions . A t th i s po in t , the reac t ion has reached dynamic
equ i l i b r ium. Cons ider the fo l low ing reac t ion :
HA (aq) + H 2 O ( l iq . ) <−> H 3 O + (aq ) + A - (aq)
Under such equ i l i b r ium cond i t ions , the to ta l concen t ra t ions o f
each spec ies rema in cons tan t , even though the spec ies in so lu t ion
a re cons tan t l y d issoc ia t ing and recombin ing . The ion iza t ion cons tan t
o f the weak monopro t i c ac id i s used to charac te r i ze the ac id , and i s
ca lcu la ted by us ing the fo l low ing equa t ion :
Ka = [H3O + ] [A - ] . . . . . . . . . . . . . . . . . . . . . . . . . (1 )
[HA]
In th is express ion , Ka re fe rs to ac id ion iza t ion cons tan t . S t rong
ac ids t yp ica l l y d issoc ia te comple te ly , and there fo re wou ld have a Ka
va lue o f g rea te r than 1 . Weak ac ids have Ka va lues much sma l le r
than 1 , t yp ica l l y less than 10 - 4 t o be more p rec ise . For ins tance , the
Ka va lue o f p ropano ic ac id i s 1 .3 x 10 - 5 and i t s pKa va lue i s 4 .874 .
In p r io r i t i z ing the conven ience , sc ien t i s ts o f ten use the pKa va lues
o f weak ac id , as i t a l lows them to dea l w i th who le numbers .
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By assuming a 1 :1 mo la r ra t io o f ac id : base , the vo lume as
we l l as concen t ra t ion o f sod ium hydrox ide , wh ich i s the base can be
used to de te rmine the number o f mo les o f ac id p resen t :
Ƞ b a s e = mass b a s e . . . . . . . . . . . . . . . . . . . . . . (2 )
M r b a s e
Ƞ a c i d = Ƞ b a s e x 1 mo l o f ac id . . . . . . . . . . . . . . . . . . (3 )
1 mo l o f base
The pH o f a so lu t ion i s re la ted to the hydron ium ions , H3O+,
concen t ra t ion by the equa t ion :
pH = − log [H3O + ] . . . . . . . . . . . . . . . . . . . . (4 )
and the pKa o f an ac id i s s imp ly :
pKa = − log Ka . . . . . . . . . . . . . . . . . . . . . . . . . . . (5 )
By cons ider ing equa t ion (1 ) , take – log on bo th s ides :
- log Ka = - log [H3O + ] – log [A - ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (6 )
[HA]
Thus , by subs t i tu t ing equa t ion (6 ) in to equa t ion (5 ) :
pKa = - l og [H3O + ] – log [A - ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (7 )
[HA]
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and there fo re , by subs t i tu t ing equa t ion (4 ) in to equa t ion (7 ) :
pKa = pH – log [A - ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (8 )
[HA]
However , s ince pKa i s cons tan t and pH var ies ,hence :
pH = pKa + log [A - ] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (9 )
[HA]
There fo re , when [A - ] = [HA] , thus :
pH a t ha l f -equ iva lence po in t = pKa . . . . . . . . . . . . . . . . . . . . . . . . (10 )
Th is i s the midway po in t to the equ iva lence po in t . A t i t ra t ion
curve i s needed to ana lyze the ac id ion iza t ion cons tan t , Ka va lues o f
the unknown monopro t i c ac id to be iden t i f i ed in th is exper iment . The
t i t ra t ion curve represen ts the pH va lues a t par t i cu la r vo lume o f
sod ium hydrox ide be ing added. Thus , pKa va lues can be read
d i rec t l y f rom t i t ra t ion curve . F rom the t i t ra t ion da ta co l lec ted f rom
the exper iment conduc ted , th ree t i t ra t ion curves o f pH versus vo lume
o f base added in m i l l i l i t res a re p lo t ted . On each curve , the vo lume o f
base a t equ iva lence po in t i s c lear l y marked . Nex t , the vo lume o f
base a t ha l f -equ iva lence po in t a t each curves a re de te rmine and
c lear l y marked . The pH va lues a t tha t par t i cu la r ha l f -equ iva lence
po in t a re then ex t rapo la ted . The pH va lues can now be used to
ca lcu la te the ac id ion iza t ion cons tan t and the average va lue i s
de te rmined . In add i t ion , the vo lume o f base t i t ra ted a t the
equ iva lence po in t i s used to ca lcu la te the concen t ra t ion o f the
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unknown monopro t i c ac id . The average i s de te rmined as we l l . The
average ac id ion iza t ion cons tan t i s then used to iden t i f y the unknown
monopro t i c ac id .
APPARATUS / REAGENTS
~ 50-mi l l i l i t re bure t te , 10-mi l l i l i t re g radua ted cy l inders , 50 and 250-
mi l l i l i t re beakers , E r lenmeyer f l ask , re to r t s tand and bure t te c lamp,
pH meter , spa tu la , ana ly t i ca l ba lance , f i l t e r funne l , magnet i c s t i r re r ,
pe l le ts o f sod ium hydrox ide , d is t i l l ed wate r , 10 mi l l i l i t res o f unknown
monopro t i c ac id .
EXPERIMENTAL PROCEDURE
1. An approx imate ly 2 .0 g rams o f pe l le ts o f sod ium hydrox ide , NaOH
is we igh ted to the neares t fou r dec ima l po in ts and d isso lved in 500
mi l l i l i t res o f d is t i l l ed wate r .
2 . A bure t te i s c leaned , r insed and f i l l ed w i th NaOH so lu t ion .
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3. 10 mi l l i l i t res o f unknown monopro t i c ac id i s p repared and then
t rans fe r red in to the Er lenmeyer f l ask .
4 . The f lask i s then p laced on a s t i r p la te and a magnet i c s t i r re r i s
inser ted in to the f lask and le t to s t i r the so lu t ion .
5 . A pH meter i s ca l ib ra ted us ing bu f fe r so lu t ion be fo re the
e lec t rode i s be ing r insed we l l w i th d is t i l l ed wate r and b lo t ted d ry .
6 . The pH e lec t rode i s inser ted in to the f lask and the pos i t ion o f the
e lec t rode i s ad jus ted so tha t the magnet i c s t i r re r does no t h i t i t .
7 . The ac id i s t i t ra ted and the pH read ing as recorded w i th the
fo l low ing inc rements :
A t pH 1 un t i l 5 .5 : 1 m i l l i l i t re o f NaOH a t a t ime
At pH 5 .5 un t i l 10 .5 : 0 .5 m i l l i l i t re o f NaOH a t a t ime
At pH 10 .5 un t i l 12 .5 : 1 m i l l i l i t re o f NaOH a t a t ime
8. The t i t ra t ion i s con t inued to a t leas t a pH o f 12 .
9 . The da ta o f pH read ings and the vo lume o f NaOH so lu t ion added
and t i t ra ted a re recorded and tabu la ted .
10 . A t i t ra t ion curve o f pH versus vo lume o f NaOH so lu t ion t i t ra ted i s
p lo t ted . Based on the t i t ra t ion curve , pKa va lue o f the ac id i s
ca lcu la ted and the ac id i s iden t i f i ed .
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RESULTS
Mass o f sod ium hydrox ide pe l le ts d isso lved : 2 .0823 g rams
Vol . NaOH
(mL)
pH
(Tr ia l 1 )
pH
(Tr ia l 2 )
pH
(Tr ia l 3 )
Average
pH
0.0 2 .89 2 .87 2 .98 2 .91
1 .0 4 .08 3 .70 3 .67 3 .82
2 .0 4 .23 4 .09 4 .05 4 .12
3 .0 4 .42 4 .32 4 .26 4 .33
4 .0 4 .68 4 .55 4 .53 4 .59
5 .0 4 .85 4 .79 4 .72 4 .79
6 .0 5 .17 5 .04 5 .02 5 .08
7 .0 5 .89 5 .46 5 .43 5 .59
7 .5 6 .33 6 .11 5 .90 6 .11
8 .0 11 .17 10 .63 10 .56 10 .79
9 .0 11 .55 11 .34 11 .26 11 .38
10 .0 11 .73 11 .52 11 .52 11 .63
11 .0 11 .83 11 .64 11 .67 11 .71
12 .0 11 .90 11 .72 11 .77 11 .80
13 .0 11 .93 11 .77 11 .82 11 .84
14 .0 12 .00 11 .80 11 .87 11 .89
15 .0 12 .04 11 .84 11 .90 11 .93
Volume of NaOH at equiva lence point (mL) 7.5951
pH at ha l f -equiva lence point ≈ pKa 4.537
Acid ion izat ion constant , Ka 2.90 x 10 - 5
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0 2 4 6 8 10 12 14 160123456789
10111213
Titration curve of pH versus Volume of NaOH (mL)pH
Vol. of NaOH (mL)
←equivalence point
←half-equivalence point
SAMPLE CALCULATION
By us ing equa t ion (2 ) :
Ƞ b a s e = mass b a s e
M r b a s e
= 2 .0823 g
39 .997 g /mo l
= 0 .0521 mo l
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From the reac t ion equa t ion , number o f mo les o f base i s equ iva len t to
the number o f mo les o f ac id . Thus ,
Ƞ a c i d = Ƞ b a s e
= 0 .0521 mo l
By us ing equa t ion (10) , pH a t ha l f -equ iva lence po in t = pKa = 4 .535
To ca lcu la te the ac id ion iza t ion cons tan t , by us ing equa t ion (5 ) :
pKa = − log Ka
4 .537 = - log Ka
Ka = a rc log -4 .535
= 2 .90 x 10 - 5
L ikewise , f rom the reac t ion equa t ion ,
HA (aq) + H 2 O ( l iq . ) <−> H 3 O + (aq ) + A - (aq)
by cons ider ing the fo l low ing “ ICE” tab le :
components HA H 3 O + A -
I n i t i a l
c o n c e n t r a t i o n
0.1 0 0
C h a n g e i n
c o n c e n t r a t i o n
0.1 – x + x + x
C o n c e n t r a t i o n a t
E q u i l i b r i u m
0.1 - x x x
the concen t ra t ion o f H 3 O + i s ca lcu la ted us ing the average va lue o f
pH read ing f rom the t i t ra t ion conduc ted in th ear l ie r exper iment .
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pH = - log [H 3 O + ]
2 .91 = - log [H 3 O + ]
[H 3 O + ] = a rc log ( -2 .91)
= 1 .23 x 10 - 3
Thus , the ac id ion iza t ion cons tan t , Ka o f the unknown monopro t i c
ac id i s d te rmine by us ing equa t ion (1 ) :
Ka = x .x__
0 .1 – x
= x 2 ___
0 .1 – x
= (1 .23 x 10 - 3 ) 2
0 .1 – 1 .23x10 - 3
= 1 .53 x 10 - 5
Thus , the pKa i s equa l to :
pKa = - log Ka
= - log (1 .53 x 10 - 5 )
= 4 .81
SAMPLE ERROR CALCULATION
The unknown monopro t i c ac id i s iden t i f i ed to be ace t i c ac id .
However , theore t i ca l l y , the pKa va lue fo r ace t i c ac id i s 4 .75 ,
whereas the Ka va lue cor responds to the unknwon monopro t i c ac id
f rom the t i t ra t ion i s 1 .76 x 10 - 5 . Thus , the percen tage e r ro rs fo r each
theore t i ca l va lue to the ca lcu la ted va lue a re :
Percen tage e r ro r (pKa) = │4 .75 – 4 .537 │ x 100%
4 .75
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= 4 .48 %
Percen tage e r ro r (Ka) = │ 1 .76 x 10 - 5 - 2 .90 x 10 - 5 │x100%
1 .76 x 10 - 5
= 64 .77 %
For the iden t i f y ing the unknown ac id us ing the pH va lue in o rder to
de te rmine the Ka va lue , the va lue i s s l igh t l y d i f fe ren t f rom the one
ob ta ined f rom the t i t ra t ion p rocess . The percen tage e r ro r i s
ca lcu la ted to be :
Percen tage e r ro r (pKa) = │4 .75 – 4 .81│ x 100%
4 .75
= 1 .26%
Percen tage e r ro r (Ka) = │1 .76x10 - 5 – 1 .53x10 - 5 │ x100%
1.76x10 - 5
= 13 .1%
DISCUSSION
A lo t o f i n fo rmat ion i s requ i red in o rder to assure tha t the
iden t i t y o f the unknown ac id i s conc lus ive . No t i ce tha t one
equ iva lence po in t i s ob ta ined , there fo re one pKa va lue . A f te r
ana lyz ing i t as we l l the Ka va lue , i t i s conc luded to be remarkab ly
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s im i la r to ace t i c ac id . The unknown ’s Ka va lue i s 2 .90 x 10 - 5 whereas
ace t i c ac id ’s Ka i s mere ly d i f fe ren t wh ich va lue i s 1 .76 x 10 - 5 .
As fo r the pKa va lue o f the unknown ac id i s ca lcu la ted to be 4 .537
whereas ace t i c ac id ’s pKa va lue i s 4 .75 .
The ma in ob jec t i ve o f the exper iment i s to de te rmine the Ka
va lue o f the unknown monopro t i c ac id so as to iden t i f y the ac id .
However , an e r ro r o f 64 .77 percen ts a f te r compar ing the theore t i ca l
va lue w i th the exper imenta l va lue i s ob ta ined . There fo re , a few
mis takes o r lack o f awareness o f the p recau t ions tha t mus t be
cons idered when conduc t ing the exper iments may be the reasons
tha t lead to the e r roneous ca lcu la t ions .
F i rs t e r ro r tha t m igh t a f fec t the ca lcu la ted va lues i s done
dur ing we igh ing the pe l le ts o f sod ium hydrox ide . The we igh t o f an
empty beaker shou ld have been cons idered as we l l . Then , i t mus t be
sub t rac ted f rom the we igh t o f the beaker con ta in ing the pe l le ts .
Hence , the t i t ra t ion curves migh t no t have the accura te va lues as the
concen t ra t ion o f sod ium hydrox ide i s no t per fec t l y 0 .1 M.
Second, the pH meter tha t has been used has never g iven the
de f in i te read ings , as the va lues tha t a re shown are a lways chang ing
rap id ly . Thus , one can never te l l t he accura te read ings o f pH va lues .
There fo re , these w i l l a l so a f fec t the t i t ra t ion curves as we l l as pKa
va lues wh ich cor respond to the pH va lues a t ha l f equ iva lence po in t .
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Bes ides tha t , the equ iva lence po in t i s no t necessar i l y be ing a t
pH o f 7 as i t occurs jus t when the concen t ra t ion o f ac id i s equa l to
the concen t ra t ion o f base reac ted in so lu t ion . There fo re , the f ina l pH
depends on the ma jo r spec ies o f ions le f t i n the so lu t ion a f te r the
reac t ion .
In add i t ion , the pH e lec t rode migh t have come in to con tac t w i th
the magnet i c s t i r re r . There fo re , a p rob lem migh t be encounte red
dur ing the read ings o f pH va lues on the pH meter .
CONCLUSION
The exper imenta l va lues o f pKa and Ka o f the unknown ac id a re
4 .537 and 2 .90 x 10 - 5 respec t i ve ly . Whereas , the pKa and Ka va lues
ob ta ined f rom us ing the in i t i a l pH o f the unknown ac id a re 4 .81 and
1 .53 x 10 - 5 respec t i ve ly . Theore t i ca l l y , the pKa and Ka va lues o f the
unknown monopro t i c ac id , wh ich i s be l ieved to be o f ace t i c ac id a re
4 .75 and 1 .76 x 10 - 5 . There fo re , an e r ro r o f 4 .48 percen ts o f the pKa
va lue and 64 .77 percen ts o f the Ka va lue f rom the t i t ra t ion p rocess
a re ca lcu la ted . Meanwh i le , an e r ro r o f 1 .26% and 13 .1% o f pKa and
Ka va lues ob ta ined f rom us ing the in i t i a l pH o f the unknown ac id i s
ca lcu la ted . There fo re , the re i s on ly a s l igh t d i f fe rence f rom the
theore t i ca l va lue compared to the va lues f rom th e t i t ra t ion p rocess .
Hence , the unknown monopro t i c ac id i s iden t i f i ed as ace t i c ac id and
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t he va lue de te rmined f rom us ing the in i t i a l pH va lue o f the ac id i s a
more accura te method .
RECOMMENDATIONS
There a re a few recommendat ions , and p recau t ions tha t have to
be cons idered dur ing the exper iments in o rder to ge t an accura te
va lue and read ings o f da ta .
F i rs t l y , the s tandard so lu t ion tha t i s used shou ld be a hundred
percen t pure and s tab le a t room tempera tu res . Thus , i t i s more
p re fe rab le to use a d r ied s tandard mate r ia l be fo re we igh ing and
d i lu ted .
Second ly , i n o rder to be more conc lus ive in iden t i f y ing the
unknown monopro t i c ac id , the mo lecu la r we igh t o f the ac id shou ld be
cons idered as we l l . Th is then can be used to compare i t w i th the
theore t i ca l va lue o f mo lecu la r we igh t o f ace t i c ac id . There fo re , i t i s
more p re fe rab le to ob ta in the mass o f so l id ac id and then on ly i t i s
d i lu ted and t i t ra ted .
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REFERENCES
~ S teven L . Murov , 2004 , Exper iments in Genera l Chemis t ry , 4 t h
Ed i t ion , Un i ted S ta tes : Thomson/Brooks /Co le .
~ h t tp : / /en .w ik iped ia .o rg /w ik i /Ac id -base_ t i t ra t ion
~ h t tp : / / chemlab .com/Chemis t ry _3_5_-
_Monopro t i c_and_Po lypro t i c_Ac ids_-_Chemis t ry
~ h t tp : / /www. t i t ra t ionexper iment .com/h tml /
APPENDICES
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Table 1: Weak Acids , K a , and pK a va lues
Ac i d HA A - K a pKa
A c e t i c C H 3 C O O H C H 3 C O O - 1 . 7 6 x 1 0 - 5 4 . 7 5
A m m o n i u m N H 4+ N H 3 5 . 6 x 1 0 - 1 0 9 . 2 5
B e n z o i c C 6 H 5 C O O H C 6 H 5 C O O - 6 . 4 6 x 1 0 - 5 4 . 1 9
C a r b o n i c H 2 C O 3 H C O 3- 4 . 3 x 1 0 - 7 6 . 3 7
H C O 3- C O 3
2 - 4 . 8 x 1 0 - 1 1 1 0 . 3 2
C h l o r o a c e t i c C H 2 C l C O O H C H 2 C l C O O - 1 . 4 x 1 0 - 3 2 . 8 5
C i t r i c C 6 O 7 H 8 C 6 O 7 H 7- 7 . 4 1 x 1 0 - 4 3 . 1 3
C 6 O 7 H 7- C 6 O 7 H 6
2 - 1 . 7 4 x 1 0 - 5 4 . 7 6
C 6 O 7 H 62 - C 6 O 7 H 5
3 - 3 . 9 8 x 1 0 - 7 6 . 4 0
F o r m i c H C O O H H C O O - 1 . 7 7 x 1 0 - 4 3 . 7 5
P h o s p h o r i c H 3 P O 4 H 2 P O 4- 7 . 5 2 x 1 0 - 3 2 . 1 2
H 2 P O 4- H P O 4
- 2 6 . 2 3 x 1 0 - 8 7 . 2 1
H P O 42 - P O 4
3 - 2 . 2 x 1 0 - 1 3 1 2 . 6 7
Refer to the a t tachment p rov ided on the nex t page .
18 | P a g e