abstract (lab 2)ionization constant

8/18/2019 Abstract (Lab 2)ionization constant

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ABSTRACT

Acids is defined as substances that ionize in water to produce H+ ions and bases is

defined as substances that ionize in water to produce OH- ions. Acids and bases are often

defined as being “weak” or “strong”. The purpose of this experiment are to determine the

acid ionization constants ! a of weak acid so"utions b# titration with sodium h#droxide

$aOH so"ution and to identif# the unknown weak acid so"utions from the ca"cu"ated !a

%a"ues obtained from the experiment. The experiment was conducted b# using titration

method. &irst"# '.( ) of sodium h#droxide so"ution was poured into the burette. After that

(' m* of unknown acid is poured into beaker and it wi"" be titrated with sodium h#droxide

so"ution to make it neutra". The pH of unknown acid is recorded b# using pH meter. &rom the

titration cur%e !a and p!a %a"ues can be determined in order to know the unknown weak acid used in this experiment. The resu"t obtained from the three tria"s of titration show the

same resu"ts which is for !a the %a"ue is .'(x (' -, whi"e p!a is .. Therefore based on the

tab"e of the acids with !a and p!a %a"ues it can be conc"ude that the unknown weak acid is

p#ridinium ion acid because in this experiment p!a and !a %a"ue ha%e the nearest %a"ue with

the p#ridinium ion acid.

INTRODUCTION

Acids and bases same as aspirin and mi"k of magnesia a"beit man# peop"e do not know

their chemica" names- acet#"sa"ic#"ic acid known as aspirin/ and magnesium h#droxide

known as mi"k of magnesium/.

Acids is defined as substances that ionize in water to produce H+ ions and bases is

defined as substances that ionize in water to produce OH- ions. Acids and bases are often

defined as being “weak” or “strong”. 0hi"e this c"assification seems somewhat arbitrar#

other more 1uantitati%e descriptors exist. &or acids !a %a"ues are common"# used. The

ionization of an acid can be shown b# the fo""owing e1uation2

HAa1/ ↔ H+

a1/ + A-a1/ 31. (/

4ince an e1ui"ibrium exists an e1ui"ibrium constant !a can be written2

! a 5 6H+ 7 6A- 7 31. 8/

[HA]

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The ! a %a"ue is an indication of acid strength. The "arger the %a"ue of the !a the

stronger the acid. This %a"ue is characteristic of the acid and can be used to he"p identif# an

unknown acid. A simi"ar s#stem exists for bases ! b/.

A more practica" measure ca""ed pH had been proposed due to the concentration of H +

and OH- ions in a1ueous so"utions are fre1uent"# %er# sma"" number and therefore

incon%enient to work with. The pH of a so"ution is defined as the negati%e "ogarithm of the

h#drogen on concentration in mo"9*/2

pH 5 - "og 6HO+7 or pH 5 - "og 6H+7 --------------------- 31. /

The negati%e "ogarithm gi%es a positi%e number for pH which otherwise wou"d be

negati%e due to the sma"" %a"ue of 6H+7. &urthermore the term 6H+7 in e1uation pertains

on"# to the numerica" part of the expression for h#drogen ion concentration for we cannot

take the "ogarithm units. Thus "ike the e1ui"ibrium constant the pH of a so"ution is

dimension"ess 1uantit#.

:ecause pH is simp"# a wa# to express h#drogen ion concentration acidic and basic

so"ution at 8'; can be distinguished b# their pH %a"ues as fo""ows2

Acidic so"ution2 6H+7 < (.' x (' -= ) pH > =.''

:asic so"ution2 6OH-7 > (.' x (' -= ) pH < =.''

$eutra" so"ution2 6H+7 5 (.' x (' -= ) pH 5 =.''

$ote that pH increases as 6H+7 decreases.

4ometimes we ma# be gi%en the pH %a"ue of a so"ution and asked the H+ ion

concentration. ?n the case we need to take the anti"og of e1uation as fo""ows2

6HO+7 5 (' @ pH or 6H+7 5 (' @pH --------------------- 31. /

OBJECTIVE

The aims of the experiment are to determine the acid ionization constants ! a of weak acid

so"utions b# titration with sodium h#droxide $aOH so"ution and to identif# the unknown

acid so"utions from the ca"cu"ated !a %a"ues obtained from the experiment.

THEORY

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ArrheniusB definitions of acids and bases are "imited in that the# app"# to the a1ueos

so"ution on"#. :ronsted acid is define as a proton donor whi"e a bronsted bases is define as a

proton acceptor. According to the :ronsted-*owre# acid-base theor# the strength of an acid

is re"ated to its abi"it# to donate protons. A"" acid-base reactions are then competitions

between bases of %arious strengths for these protons. &or examp"e the strong acid H;" reacts

with water according to the e1uation2

H;(a1/ + H8O(/ HO+

a1/ + ;"-a1/

H;" is a strong acid and is comp"ete"# ionized in other words ('' percent ionized in

di"ute so"ution. ;onse1uent"# the 6HO+7 of '.( M H;" is '.( M . The H;" is a stronger acid

than

water and donates a proton to water to form HO+ .

:# contrast acetic acid ;H;OOH is a weak acid and is on"# s"ight(# dissociated as

shown in the e1uation2

;H;OOH a1/ + H8O(/ HO+

a1/ + ;H;OO-a1/

?ts acid ionization constant as shown be"ow is therefore sma""2

[ ]

3 3

a

3

5H O CH COO

K

CH COOH

1.76x10

+ −

−=

=

Acetic acid on"# partia""# dissociates in a1ueous so"ution and an appreciab"e 1uantit# of un-

ionized acetic acid remains in so"ution.

The %ast maCorit# of acids are weak acids. ;onsider a weak monoprotic acid HA. The

ionization of weak acid is represented b#

HA a1/ + H8O"/ HO+

a1/ + A-a1/

Or simp"#

HAa1/ H+

a1/ + A-a1/

The e1ui"ibrium expression for this ionization is

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! a 5

+¿

H 3O¿

¿−¿

A¿

¿

¿¿

or

+¿

H ¿

¿

−¿

A¿

¿

¿¿

where ! a is the acid ionization constant the e1ui"ibrium constant for the ionization of an acid.

At a gi%en temperature the strength of acid HA is measured 1uantitati%e"# b# the magnitude

of ! a. The "arger the %a"ue of ! a the stronger the acid wi"" be the greater the concentration of

H+ ions at e1ui"ibrium due to its ionization.

4trong acids usua""# ha%e ! a < ( where ! a is the dissociation constant. This is because it

is t#pica""# dissociate comp"ete"# whi"e weak acids tends to ha%e ! a %a"ue sma""er than (.

0hen a strong base is added to a so"ution of a weak acid the h#droxide ion reacts with some

of the HO+ present therefore disturbing the e1ui"ibrium. )ore of the acid wi"" dissociate

unti" a new e1ui"ibrium is estab"ished.

PROCEDURE

(. A burette is rinsed with tap water and then with disti""ed water. ?t is c"ean if the burette

drains without "ea%ing drops of water behind. ?f not the burette is washed with di"ute

detergent so"ution.

8. 4ome of the sodium h#droxide is poured into the beaker and this sma"" amount is

poured into the burette and "et it drain trough into a waste beaker. The burette is fi""ed

with sodium h#droxide so"ution. )ake sure the tip of the burette is comp"ete"# fi""ed.

. (' m* of the unknown acid is transferred into a beaker and enough water is added to

the beaker so that the end of the pH e"ectrode is co%ered.

. The reading of the sodium h#droxide in the burette and the pH initia" for the unknown

acid is recorded.

. The pH e"ectrode is rinsed and it is p"aced in the beaker with the acid. Dour set-up is

adCusted so that the burette containing sodium h#droxide is o%er the beaker.

,. A sma"" portion of the sodium h#droxide is added "itt"e b# "itt"e to the acid samp"e.

After each addition the tota" %o"ume of sodium h#droxide added and the pH of the

so"ution is recorded.

=. Erop b# drop of the sma""er portions of the sodium h#droxide is added when the pH

begins to rise rapid"#. The adding portion is continued unti" the pH is in the baseregion.

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F. The titrated acid samp"e in the beaker is discarded and the titration is repeated twice

more.

G. The graph of pH %s %o"ume of sodium h#droxide added is p"otted. The e1ui%a"ence

point on each graph is determined.

('. The amount of the base needed to reach ha"f wa# to the e1ui%a"ence point is

determined. The pH at this %o"ume of base is con%erted to 6H+7 to gi%e the ! a of the

acid.

APPARATUS AND MATERIAL

Apparatus Descriptions

Retort stand To hold the burette.Beaker All solutions used in this experiment were placed in

these beakers.

pH meter To measure changes of pH value of the solution.

Burette To place the NaH solution for titration.

!easuring c"linder To measure and transfer the right amount of solutions

needed from its actual container into the beaker.

!aterials

#odium h"droxide$ NaH solid.%nknown acid

Distilled water.

RESULT

Titration (

o"ume

of

$aOH

m"/

' ( 8 , = F G (' (( (8

pH

a"ues

.

'

.,

'

.G

8

.'

8

.8

.

G

.

.=

,

,.'

,

,.,

=

('.=

8

((.(8 ((.'

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Titration 8

o"ume

of

$aOH

m"/

' ( 8 , = F G G. G., G.=

6

at pH =At pH =

&igure '( Titration '


7/12

pH

a"ues

.8

=

.

=

.F

.'

(

.(

=

.

,

.

8

.=

'

.G

,.

=

=.' =.' F.(

Titration

o"ume

of

$aOH

m"/

' ( 8 , = F G G. G.,

pH

a"ues

.8

.

=

.F

'

.G

G

.(

F

.

.

8

.=

'

,.(

,.

8

=.88 =.,

7

&igure )( Titration )


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0 2 4 6 8 10 120

1

2

3

4

5

6

7

8

9

Graph of pH values versus volume of NaOH

Volume of NaOH solu!o" #m$%

pH value

O%era"" Iesu"ts

Titration ( Titration 8 Titration

o"ume of unknown

acid m*/

(' (' ('

o"ume of $aOH

so"ution at

e1ui%a"ence point/

needed to neutra"ize

the unknown acid

G.(F G. G.

8

&igure *( Titration *


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m*/

o"ume of ha"f

e1ui%a"ence point

m*/

.G .= .,

pH %a"ue at ha"f

e1ui%a"ence point

. . .

SAMPLES OF CALCULATION

HA J H+ + A-

pH 5

H ¿

+¿¿

¿

log ¿

Thus

6H+7 5 ('- pH

0here pH is at ha"f e1ui%a"ence point which is .

+¿ H

¿

¿

A

¿

−¿¿¿

¿¿

K a=¿

o"ume of sodium h#droxide to neutra"ize the unknown acid at ha"f e1ui%a"ence 5 o"ume of

sodium h#droxide to neutra"ize the unknown acid K 8. &or examp"e using titration (/

Ha"f %o"ume of e1ui%a"ence point 5 G.(F m* K 8

5 .G m*

At ha"f e1ui%a"ence point exact"# ha"f the acid has been con%erted into its conCugate base so

at this point 6H+7 5 6A-7. Therefore2-

H ¿

+¿¿

K a=¿

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And

pK a= pH

Titration pH at ha"f

e1ui%a"ence

p!a ! a

( &rom the figure (

pH at ha"f

e1ui%a"ence 5 .

p! a 5 pH at ha"f

e1ui%a"ence

Therefore p! a 5 .

6H+7 5 ('- pH

5 ('-.

5 .'( x ('-,

Thus

! a 5 6H+7

5 .'( x ('-,

8 &rom the figure 8

pH at ha"f

e1ui%a"ence 5 .

p! a 5 pH at ha"f

e1ui%a"ence

Therefore p! a 5 .

6H+7 5 ('- pH

5 ('-.

5 .'( x ('-,

Thus! a 5 6H

+7

5 .'( x ('-,

&rom the figure

pH at ha"f

e1ui%a"ence 5 .

p! a 5 pH at ha"f

e1ui%a"ence

Therefore p! a 5 .

6H+7 5 ('- pH

5 ('-.

5 .'( x ('-,

Thus

! a 5 6H+7

5 .'( x ('-,

DISCUSSION

This is experiment was conducted to ca"cu"ate the p!a and !a %a"ue. The p!a and !a

%a"ues shou"d be known in order to identif# the unknown acid used in this experiment. This

experiment was conducted b# using titration method. Titration method is a techni1ue where a

so"ution of known concentration is used to determine the concentration of an unknown

so"ution. T#pica""# the titrant the know so"ution/ is added from a burette to a known 1uantit#

of the ana"#te the unknown so"ution/ unti" the reaction is comp"ete. !nowing the %o"ume of

titrant added a""ows the determination of the concentration of the unknown. Often an

indicator is used to usua""# signa" the end of the reaction the endpoint. :urette is used as an

important apparatus for titration method. A t#pica" burette has a sma"" ca"ibration unit of '.(

m* therefore %o"ume dispense from the burette shou"d be estimated to the nearest '.'( m*.

&rom the &igure ( 8 and it shows the graph of pH %a"ue %ersus %o"ume of sodium

h#droxide so"ution used in the experiment. &rom the &igure ( 8 and the red "ine indicate

the e1ui%a"ence point the point where the unknown acid start to neutra"ize/ and the green

"ine indicate the ha"f e1ui%a"ence point. The %o"ume of sodium h#droxide so"ution needed to

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neutra"ize the unknown acid is G.(F m* G. m* and G. m* respecti%e"# for each figure. &or

figure ( shows an increasing pattern but not %er# smooth. This is because when the unknown

acid has been neutra"izing it wi"" gi%e a sudden change in the pH which is from the %a"ue of

,.,= to ('.=8. Therefore it has a "itt"e unstab"e increasing graph pattern.

?n order to identif# the unknown acid the p!a and !a %a"ue shou"d be ca"cu"ated. ?n

order to ca"cu"ate the p!a and !a %a"ue we need to know the pH %a"ue at ha"f e1ui%a"ence

point. The ha"f e1ui%a"ence point is got from the %o"ume of sodium h#droxide so"ution

needed to neutra"ize the unknown acid di%ided b# two. &rom the figure ( 8 and the ha"f

e1ui%a"ence point are .G m* .= m* and ., m* respecti%e"#.

&rom the ca"cu"ation shown abo%e when ha"f e1ui%a"ence point is known !a is e1ua" to

the 6H+7 because in ha"f e1ui%a"ence point exact"# ha"f the acid has been con%erted into its

conCugate base so at that point 6H+7 is e1ua" to 6A-7. pH can be ca"cu"ated from the "og 6H+7.

:ased on the three graph abo%e the pH at e1ui%a"ence point for each titration is ..

According to the formu"a p!a 5 pH therefore there p!a %a"ue has the same %a"ue with pH

%a"ue which is .. The %a"ue of !a can be obtained b# using formu"a pH 5 "og 6H+7 !a

%a"ue is e1ua" to 6H+7. The !a %a"ue for the three titration is .'( x (' -,. &rom the tab"e of !a

it shows that the unknown acid is known as p#ridinium ion acid.

CONCLUSION

As for the conc"usion the experiment was successfu""# conducted. The unknown acid

used in this experiment can be determine. The p!a and !a %a"ues a"so can be determined.

&rom the !a %a"ue and p!a %a"ue obtained from the ca"cu"ation it can be conc"uded when !a

is .'( x ('-, and p!a is . the unknown acid is known as p#ridinium ion acid which it has a

near %a"ue of p!a and !a with this experiment.

RECOMMENDATION

(. :efore the experiment started pour sma"" amount of sodium h#droxide so"ution into

the burette before adding a who"e burette with sodium h#droxide so"ution in order to

make sure there is no other substance or materia"s in the beaker that can affected the

resu"t.

8. )ake sure there is no bubb"e present in the burette before start the titration process to

get accurate %o"ume of sodium h#droxide.

. The pH meter must be ca"ibrated first before been used in order to obtain the precise

resu"t.

. &or e%er# pH reading the pH rod must be rinsed with disti""ed water to remo%e an#

impurities.

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. The pH wi"" change rapid"# when it near the e1ui%a"ent point in order to pre%ent an#

error the titration process must be done s"ow"# through this point.

REFERENCE

'. +hemical laborator" manual +H,-/$ facult" of chemical engineering$ %0T!

#hah Alam.

). +hang$ R.$ 1oldsb"$ 2. A.$ +hemistr" 3''th edition4$ Acid5base e6uilibria and

solubilit" e6uilibria 3p g 7))4.

*. 8hat is titration9 Retrieved from

http(::chemed.chem.purdue.edu:genchem:lab:techni6ues:titration:what.html$

ctober '/$ );'/.

-. pH and p2a. Retrieved from http(::chemistr".tutorvista.com:inorganic5

chemistr":ph5and5pka.html$ ctober '7$ );'/.

APPENDICES

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abstract (lab 2)ionization constant

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