39166239 Identifying an Unknown Weak Acids Experiment
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Transcript of 39166239 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 .
pH = - log [H 3 O + ]
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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
= 4 .48 %
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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
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
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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 ac id 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 .
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
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t he 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
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 .
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RECOMMENDATIONS
There are a few recommendations, and precautions that have to be considered during the experiments in order to get an accurate value and readings of data.
Firstly, the standard solution that is used should be a hundred percent pure and stable at room temperatures. Thus, it is more preferable to use a dried standard material before weighing and diluted.
Secondly, in order to be more conclusive in identifying the unknown monoprotic acid, the molecular weight of the acid should be considered as well. This then can be used to compare it with the theoretical value of molecular weight of acetic acid. Therefore, it is more preferable to obtain the mass of solid acid and then only it is diluted and titrated.
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
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~ 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
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
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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 .
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