11-lecture 11 (2015-07-21) (student version) (1).ppt
TRANSCRIPT
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1
Housekeeping for Tue. 7/21/15- Exam 3 graded; scores posted to Back!oard; "i return at end of cass- Exam 2 corrections due tomorro" #$ed. 7/22%
- H$ 13 #and soutions% posted to Back!oard and &ooge 'ri(e- ) a of acetic acid "ork-up due toda*+
- Exam 3 corrections in pa*; due in cass next Monday #7/27%+
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Titration
, in an acid-!ase titration a soution of kno"n concentration
#titrant% is so"* added to a soution of unkno"n concentration#analyte% from a !urette unti te reaction is compete
"en te reaction is compete "e a(e reaced te endpoint ofte titration
,an indicator ma* !e added to determine te endpoint an indicator is a cemica tat canges coor "en te pH
canges
, "en te moes of H301 moes of 0H te titration as reaced
its equivalence point
, E4ui(aence point not te same as end point+
E4ui(aence point is te theoretical point "en moes acid moes !ase.
End point is experimental point tat approximates e4ui(aence point.
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Titration
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6ndicators
, man* d*es cange coor depending on te pH of te soution
, tese d*es are "eak acids esta!ising an e4uii!rium "it te H20
and H301 in te soution
H-6nd#aq% H20#l %⇔ 6nd−#aq% H301#aq%
) a 6nd-8H3018 / H-6nd8
• p) a of indicator is pH "ere indicator canges coor
• $en using an indicator to monitor a titration "e "ant to matc te
p) a of te indicator "it te pH of te e4ui(aence point as cose*
as possi!e.
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5
9enoptaein
5
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:
et* > > >
H
>a00=
=
= =H
=H
=H
=H
=
=H
=H
=
=H
=H
#=H3%
2 > > > >
>a00=
H301 0H-
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atcing indicators "it titrations
Te e4ui(aence point for te titration of H= "it >a0H occurs at pH 7.
$ic indicators "oud !e suita!e for tis titration?
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T*pes of titrations
@our genera t*pes of titrationsA
- strong acid / strong !ase #!ase added to acid; !ase titrant%
- strong !ase / strong acid #acid added to !ase; acid titrant%
- "eak acid / strong !ase
- "eak !ase / strong acid
titrations sare common traitsC
- pH !efore titrant is added
- pH !efore e4ui(aence point is reaced
- pH at e4ui(aence point #acid or !ase is entire* neutraiDed+%
- pH after e4ui(aence point #excess titrant%
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trong acid / strong !ase titration #>a0H added to H=%
0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
initia pH
pH !efore e4ui(aence point
e4ui(aence
point
pH after e4ui(aence point
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 1A $at is te initia pH?
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
uick detourC Gooking at te titration cur(e "e can see tat "e
a(enIt reaced te e4ui(aence point. But "e usua* donIt a(e a
titration cur(e to refer to. 6n fact "e donIt rea* need te titrationcur(e to gauge "ere "e are in te titration cur(e.
$at "e need to kno" isC
- te tota moes of acid !efore "e start to titrate
- te (oume of !ase "e need to add to reac te e4ui(aence point
#in oter "ords te (oume of !ase to react compete* "it a te
acid%
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 3A $at is te pH at te e4ui(aence point?
0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase A $at is te pH "en 3FF.F mG #tota% of >a0H a(e !eenadded?
0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
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trong acid / strong !ase titration #>a0H added to H=%
Jo. >a0H #mG% pH
F F.3
1FF F.7K
2FF 7
3FF 13
35F 13.
Tings to keep in mind "en "orking a titration pro!emC
- tota L of moes of acid #or !ase%
- L of moes of acid #or !ase% eft after addIn of titrant; eps *ou
decide if *ouI(e reaced te e4ui(aence point *et
- at e4ui(aence point no moes of acid #or !ase% eft+
- after e4ui(aence point pH is due to excess titrant
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0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
$eak acid / strong !ase titration #>a0H added to H=2H302%
initia pH
pH !efore e4ui(aence point; *buffering region!*
e4ui(aence
point
pH after e4ui(aence point
1:
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$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
9ase 1A $at is te initia pH?
17
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$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
9ase 2A $at is te pH "en 1FF mG of >a0H are added?
0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
1K
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$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
1M
9ase 2A $at is te pH "en 1FF mG of >a0H are added?
uick detourC Gooking at te titration cur(e "e can see tat "e
a(enIt reaced te e4ui(aence point. gain "e usua* donIt a(e
a titration cur(e to refer to. nd again "e donIt rea* need te
titration cur(e to gauge "ere "e are in te titration cur(e.
$at "e need to kno" isC
- te tota moes of acid !efore "e start to titrate
- te (oume of !ase "e need to add to reac te e4ui(aence point#in oter "ords te (oume of !ase to react compete* "it a te
acid%
- this is exactly the same as what we did with the strong acid /
strong base titration
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$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
9ase 2A $at is te pH "en 1FF mG of >a0H are added?
0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
Haf-e4ui(aence
point+
t af-e4ui(aance point acid8 conNugate !ase8.
pH p) a+
Buffering region is p) a
/- 1.
Buffering region
2F
k id / ! i i # dd d %
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$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
9ase 3A $at is te pH at te e4ui(aence point?
0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
E4ui(aence point
21
k id / ! i i # 0 dd d = 0 %
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$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
9ase 3A $at is te pH at te e4ui(aence point?
0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
E4ui(aence point
>otice tat te pH is not 7+ Tis
is !ecause of te e4uii!rium of
acetate as a "eak !ase at te
e4ui(aence point.
22
$ k id / ! i i #> 0H dd d H= H 0 %
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9ase A $at is te pH "en 3FF.F mG #tota% of >a0H a(e !een
added?
$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
0
2
4
6
8
10
12
14
0 100 200 300 400
Vol. NaOH (mL)
p H
23
$ k id / ! i i #> 0H dd d H= H 0 %
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Jo. >a0H #mG% pH
F 2.521FF .7
2FF M.1
3FF 13
35F 13.
Tings to keep in mind for a "eak acid/strong !ase titrationC
- tota L of moes of acid #or !ase%
- "at e4uii!rium is taking pace in te !uffer region? $eak acid or "eak !ase?
- "at e4uii!rium is taking pace at te e4ui(aence point? $eak
!ase or "eak acid?
- !e*ond e4ui(aence point cacuations are same as strong acid/
strong !ase titration
$eak acid / strong !ase titration #>a0H added to H=2H302%
2
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25
atcing indicators "it titrations
Te e4ui(aence point for te titration of H=2H302 "it >a0H occurs at O
pH M. $ic indicators "oud !e suita!e for tis titration?
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outions to exercises
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 1A $at is te initia pH?
What’s going on?
Te initia pH is te pH of te soution !efore "e add an* titrant.
ince H= is a strong acid te pH is Nust te *dronium
concentration.
H308 H=8 F.5
pH -og #F.5 %
0.
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
What’s going on?
Before "e start our cacuations etIs Nust do a 4uick detour to
determine te total num!er of moes of acid tat *ouIre starting"it. Tis "i te us t"o tingsA
- "at (oume of !ase "e need to reac te e4ui(aence point;
- o" muc acid is eft o(er after "eI(e added our 1FF mG of !ase6nitia moes of H= H=8 x Jo. H=
#F.5 moes / G% x #F.2FF G%
F.1 moes 1FF mmoes
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
What’s going on?
rigt so o" muc !ase do "e need to add to react
compete* "it 1FF mmoes of our acid? Te neutraiDationreaction so"s tat >a0H reacts 1A1 "it H=C
H= >a0H→ >a= H20
o "e need 1FF mmoes of !ase. $eI use te moarit* of >a0H todetermine te (oumeCF.1FF moes >a0H x #1 G / F.5 moes% F.2 G 00 m"
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
What’s going on?
rigt+ >o" "eIre read* to cacuate te pHC
@rom te pre(ious cacuation "e determined tat "e need to add
2FF mG of our !ase to reac te e4ui(aence point. 6n pase 2
"eI(e on* added 1FF mG of !ase. #o "e a(enIt reaced te
e4ui(aence point. $eI sti a(e H= eft o(er.
#o pH "i !e due to te *dronium from te remaining H=.
Ho" muc acid "i !e eft o(er?
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
$alculations%
oes of >a0H added >a0H8 x Jo. >a0H
#F.5 moes / G% x #F.1FF G%
F.F5 moes 5F mmoes
ince H= reacts "it >a0H in a 1A1 manner 5F mmoes of H=
are neutraiDed.
oes H= remaining 1FF mmoes - 5F mmoes 5F mmoes
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
$alculations%
0) amost tere. >o" "e Nust need to cacuate te pH !ased on
te amount of remaining *dronium.
H308 moes remaining acid / new (oume+
$e need to keep track of our (oumeA
initia (oume 2FF mG
1FF mG of >a0H added
>e" (oume #2FF 1FF% mG 00 m"
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
$alculations%
0) amost tere. >o" "e Nust need to cacuate te pH !ased on
te amount of remaining *dronium.
H308 moes remaining acid / new (oume+
5F mmoes / 3FF mG
F.17
pH -og #F.17 %
0.&'
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 3A $at is te pH at te e4ui(aence point?
What’s going on?
t te e4ui(aence point te amount of !ase is e4ua to te amount
of acid. Tat is moes !ase moes acid.
(ll the acid has been neutrali)ed!
Te on* source of acidit* is from te autoioniDation of "ater.
) " H3080H-8 1.F x 1F-1
ince H308 0H-8 for pure "ater H30
8 1.F x 1F-7
pH -og #1.F x 1F-7% &
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase 3A $at is te pH at te e4ui(aence point?
@or te titration of a strong acid "it a strong !ase #or (ice (ersa%
te pH at te e4ui(aence point "i always !e 7.
Tis is not te case for "eak acids and "eak !ases as "eI seesort*.
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase A $at is te pH "en 3FF.F mG #tota% of >a0H a(e !eenadded?
What’s going on?
t tis point "eIre adding excess !ase. Te pH "i !e due to te
excess *droxide.
Ho" muc excess *droxide do "e a(e?
Tere are t"o "a*s "e can figure tis outA
- cacuate te tota moes of !ase tat a(e !een added; su!tract
te amount of !ase needed to neutraiDe te acid
- or cacuate te (oume of !ase re4uired to neutraiDe te acid;
te excess !ase "i come from te (oume of !ase tatIs eft o(er
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase A $at is te pH "en 3FF.F mG #tota% of >a0H a(e !eenadded?
$alculationsA
-'etermining amount of excess !aseC
$e started "it 1FF mmoes of acid. o "e need 1FF mmoes of
!ase to neutraiDe te acid.
moes of !ase in 3FF mG F.5 >a0H ?
moes >a0H >a0H8 x Jo. >a0H
#F.5 moes / G% x #F.3FF G%
F.15F moes or 15F mmoes
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trong acid / strong !ase titration #>a0H added to H=%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=
9ase A $at is te pH "en 3FF.F mG #tota% of >a0H a(e !eenadded?
$alculationsA
Excess moes !ase 15F mmoes 1FF mmoes 5F mmoes
0H-8 moes excess !ase / new (oume
$e started "it 2FF mG of acid; "eI(e added 3FF mG of !aseC
new (oume #2FF 3FF% mG 00 m"0H-8 5F mmoes / 5FF mG F.1
p0H -og #F.1 % 1
pH 1 - p0H 1 - 1 +
k id / ! i i # 0 dd d = 0 %
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$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
9ase 1A $at is te initia pH?
What’s going on?
Te initia pH is te pH of te soution !efore "e add an* titrant.
ince acetic acid is a "eak acid it exists in e,uilibrium !et"een its
acid form and its conNugate !ase formA
H=2H302 H20 PQ =2H302- H30
Te pH is determined !* te e,uilibrium concentration of*dronium.
3M
$ k id / ! i i #> 0H dd d H= H 0 %
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$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
9ase 1A $at is te initia pH?
$alculations%
et up an 6=E ta!e "it F.5 as te initia concentration of acetic
acid and F as te initia concentration of acetate and *dronium
H=2H3028 =2H302-8 H30
8
6nitia F.5 F F
=ange -x x x
E4uii!rium F.5 - x x x
F
$ k id / ! i i #> 0H dd d H= H 0 %
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$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
9ase 1A $at is te initia pH?
$alculations%
et up e4uii!rium expressionA
) a =2H302-8H30
8 / H=2H3028
1.K x 1F-5 x#x% / #F.5 x%; in(oke Rx is smaS approximation
x2 F.5 x #1.K x 1F-5%
x 3 x 1F-3
H308 3 x 1F-3
pH -og H308 -og #3 x 1F-3 % .
1
$ k id / ! i i #> 0H dd d H= H 0 %
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$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
What’s going on?
Before "e start our cacuations etIs Nust do a 4uick detour to
determine te total num!er of moes of acid tat *ouIre starting"it. Tis "i te us t"o tingsA
- "at (oume of !ase "e need to reac te e4ui(aence point;
- o" muc acid is eft o(er after "eI(e added our 1FF mG of !ase
6nitia moes of H=2H302 H=2H3028 x Jo. H=2H302
#F.5 moes / G% x #F.2FF G%
F.1 moes 1FF mmoes
$ k id / t ! tit ti #> 0H dd d t H= H 0 %
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=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
What’s going on?
rigt so o" muc !ase do "e need to add to react
compete* "it 1FF mmoes of our acid? Te neutraiDationreaction so"s tat >a0H reacts 1A1 "it H=2H302C
H=2H302 >a0H→ >a=2H302 H20
o "e need 1FF mmoes of !ase. $eI use te moarit* of >a0H todetermine te (oumeCF.1FF moes >a0H x #1 G / F.5 moes% F.2 G 00 m"
$eak acid / strong !ase titration #>a0H added to H=2H302%
t id / t ! tit ti #> 0H dd d t H=%
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trong acid / strong !ase titration #>a0H added to H=%
9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
What’s going on?
rigt+ >o" "eIre read* to cacuate te pHC
@rom te pre(ious cacuation "e determined tat "e need to add
2FF mG of our !ase to reac te e4ui(aence point. 6n pase 2
"eI(e on* added 1FF mG of !ase. #o "e a(enIt reaced te
e4ui(aence point. $eI sti a(e H=2H302 eft o(er.
#o pH "i !e due to te *dronium from te remaining
H=2H302
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
$eak acid / strong !ase titration #>a0H added to H= H 0 %
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9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
What’s going on?
H=2H302 is !eing neutraiDed !* >a0HA
H=2H302 >a0H→ >a=2H302 H20
$e sti a(e H=2H302 eft o(er - !ecause "e a(enIt reaced te
e4ui(aence point.
>eutraiDation of acetic acid creates its conNugate !ase acetate.We’-e created a buffer!
*#o we can use the endersonasselbach e,uation to
determine the p!*
$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
5
$eak acid / strong !ase titration #>a0H added to H= H 0 %
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9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
$alculations%
Tota moes acetic acid acetic acid8 x Jo. cetic acid
#F.5 % x #F.2 G%
1FF mmoes
oes >a0H added >a0H8 x Jo. >a0H
#F.5 % x #F.1 G%
5F mmoes
$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
:
$eak acid / strong !ase titration #>a0H added to H= H 0 %
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9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
$alculations%
H=2H302 >a0H→ >a=2H302 H20
oes >a0H moes acetate formed moes acetic acid consumed
o moes acetate 5F mmoes
oes acetic acid remaining 1FF mmoes 5F mmoes
5F mmoes
$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
7
$eak acid / strong !ase titration #>a0H added to H= H 0 %
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9ase 2A $at is te pH "en 1FF mG of >a0H a(e !een added?
$alculations%
pH p) a og #-8 / H8%
-og #1.K x 1F-5% og #5F mmoes acetate / 5F mmoes
acetic acid%
.7 og #1%
1.&1
$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
$e can repace te concentration terms "it
L of moes !ecause te (oumes in eac of teconcentrations "i cance eac oter out+
K
$eak acid / strong !ase titration #>a0H added to H=2H302%
-
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9ase 3A $at is te pH at te e4ui(aence point?
What’s going on?
H=2H302 >a0H→ >a=2H302 H20
t te e4ui(aence point all te acid as !een neutraiDed. Tat is
a te acetic acid as !een con(erted to acetate.
2ather than pure water we ha-e a solution of a wea3 base 4the
acetate anion56
=2H302- H20 PQ H=2H302 0H
-
$eI set up an 6=E ta!e to determine te e4uii!rium
concentration of *droxide.
$eak acid / strong !ase titration #>a0H added to H=2H302%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
M
$eak acid / strong !ase titration #>a0H added to H=2H302%
-
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9ase 3A $at is te pH at te e4ui(aence point?
$alculations
=2H302- H20 PQ H=2H302 0H
-
$atIs te concentration of acetate?
oes acetate 1FF mmoes F.1 moes
Jo. oution ?
6nitia (o. 2FF mG
$at (o. of >a0H a(e "e added to get to e4ui(aence point?
>a0H8 moes >a0H / Jo. >a0H
Jo. >a0H moes >a0H / >a0H8 F.1 moes / #F.5 moes/G%
F.2 G 2FF mG
g # 2 3 2%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
5F
$eak acid / strong !ase titration #>a0H added to H=2H302%
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9ase 3A $at is te pH at te e4ui(aence point?
$alculations
=2H302- H20 PQ H=2H302 0H
-
$atIs te concentration of acetate?
oes acetate 1FF mmoes F.1 moes
Jo. oution ?
6nitia (o. 2FF mG
2FF mG of F.5 >a0H added to reac e4ui(aence point
>e" (oume 2FF mG 2FF mG 100 m"
=2H302-8 F.1 moes / F. G 0.25 M
g # 2 3 2%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
51
$eak acid / strong !ase titration #>a0H added to H=2H302%
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9ase 3A $at is te pH at te e4ui(aence point?
$alculations
=2H302- H20 PQ H=2H302 0H
-
=2H302-8 F.1 moes / F. G F.25
>o" "e can set up our 6=E ta!eC
g # 2 3 2%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
=2H302-8 H=2H3028 0H
-8
6nitia F.25 F F
=ange -x x x
E4uii!rium F.25 - x x x
52
$eak acid / strong !ase titration #>a0H added to H=2H302%
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$alculations%
Before "e set up our e4uii!rium expression "e need to remem!er
tat "eIre deaing "it te e4uii!rium of a "eak !ase in "ater.
What’s the e,uilibrium constant?
$e need te ) ! for acetate as a "eak !ase. But o" do "e get ) !?
$eI use te reationsip !et"een ) " ) a acid ) ! conNugate !ase A
) " ) aacid x ) ! conNugate !ase
) ! conNugate !ase ) " / ) a acid
) ! acetate ) " / ) a acetic acid; ) a acetic acid 1.K x 1F-5
-1 -5 -10
9ase 3A $at is te pH at te e4ui(aence point?
g # 2 3 2%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
53
$eak acid / strong !ase titration #>a0H added to H=2H302%
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$alculations%
et up e4uii!rium expressionA
) ! H=2H30280H-8 / =2H302
-8
5.: x 1F-1F x#x% / #F.25 x%; in(oke Rx is smaS approximation
x2 F.25 x #5.: x 1F-1F%
x 1.2 x 1F-5
0H-8 1.2 x 1F-5
p0H -og 0H-8 -og #1.7 x 1F-5 % .M
pH 1 - p0H 1 - .K 7.+
9ase 3A $at is te pH at te e4ui(aence point?
g # 2 3 2%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
5
$eak acid / strong !ase titration #>a0H added to H=2H302%
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9ase A $at is te pH "en 3FF.F mG #tota% of >a0H a(e !een
added?What’s going on?
t tis point "eIre adding excess !ase. Te pH "i !e due to te
excess hydroxide.
$at a!out te acetate? $* isnIt it contri!uting?
ddition of excess *droxide "i pre-ent acetate from con(erting
to its conNugate acid form. $e can expain tis using Ge =teierIs
9rincipe. =2H302
- H20 PQ H=2H302 0H
-
g # 2 3 2%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
55
$eak acid / strong !ase titration #>a0H added to H=2H302%
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9ase A $at is te pH "en 3FF.F mG #tota% of >a0H a(e !een
added?What’s going on?
ince te pH is due to te excess *droxide "e a(e te exact
same scenario as "e did "it te titration of H= "it >a0H. Te
cacuations are exact* te same. @or competeness 6I repeattem.
Ho" muc excess *droxide do "e a(e?
Tere are t"o "a*s "e can figure tis outA- cacuate te tota moes of !ase tat a(e !een added; su!tract
te amount of !ase needed to neutraiDe te acid
- or cacuate te (oume of !ase re4uired to neutraiDe te acid;
te excess !ase "i come from te (oume of !ase tatIs eft o(er
g # 2 3 2%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
5:
$eak acid / strong !ase titration #>a0H added to H=2H302%
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9ase A $at is te pH "en 3FF.F mG #tota% of >a0H a(e !een
added?$alculationsA
-'etermining amount of excess !aseC
$e started "it 1FF mmoes of acid. o "e need 1FF mmoes of !ase to neutraiDe te acid.
moes of !ase in 3FF mG F.5 >a0H ?
moes >a0H >a0H8 x Jo. >a0H
#F.5 moes / G% x #F.3FF G%
F.15F moes or 15F mmoes
g # 2 3 2%
=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302
57
$eak acid / strong !ase titration #>a0H added to H=2H302%
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9ase A $at is te pH "en 3FF.F mG #tota% of >a0H a(e !een
added?$alculationsA
Excess moes !ase 15F mmoes 1FF mmoes 5F mmoes
0H-8 moes excess !ase / new (oume$e started "it 2FF mG of acid; "eI(e added 3FF mG of !aseC
new (oume #2FF 3FF% mG 00 m"
0H-8 5F mmoes / 5FF mG F.1
p0H -og #F.1 % 1
pH 1 - p0H 1 - 1 +
g 2 3 2=ase stud*A F.5 >a0H is added to 2FF.F mG of F.5 H=2H302