determination of ka of unknown acid

8/18/2019 determination of ka of unknown acid

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Table of Content

Bil Content Page

1. Abstract 2

2. Introduction 2

3. Objective 3

4. Apparatus 3

. T!eor" 4#$

%. Procedure &

$. 'esult and Calculation (#14

&. )iscussion 1#1%

(. Conclusion 1$

1*. 'eco++endation 1&

11. 'eference 1(


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1.1 Abstract

A solution of sodium hydroxide, a strong base, will be used as the titrant to generate a pH

titration curve for an unknown acid. Measurement of the pH as the titration proceeds is

done by using a pH electrode. The initial acid concentration and the pH at the initial

point, half-euivalence point, and euivalence point will allow the calculation of values

of the !a for the acid. A plot of pH versus volume of "a#H solution added will be usedto determine the p!a of acid. Two methods of calculations are used to determine the !a

of the unknown acid which are the initial $h method and the half-euivalence method.

%rom the calculation, by using method &, the !a value is 6.383 x10−6

while method '

gives the !a value of 5.163 x 10−8

. The unknown acid is determined to be a $otassium

Hydrogen $hthalate (!H$). The percentage error for method & is *+.' while the

percentage error for method ' is .*/. 0t is proven that method & is more accurate.

However, the results for this experiment is not acceptable as it has high percentage error.

1.2 Introduction

An unknown acid with known molarity is used in titration with sodium hydroxide. A titration

is a process in which there is a small increment of a solution of known concentration are

added to a specific volume of a solution of unknown concentration until the stoichiometry for

that reaction is attained. The calculation for the value of K a can be done from the known

uantity of known solution reuired to complete the titration. The purpose of this titration isto determine the euivalence point and half-euivalence point of the reaction that can be

obtained from the graph. The euivalence point occurs when the moles of acid in solution

euals to the moles of bases added in the titration, showing that the solution have been

neutrali1ed.

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1.3 Objective

To determine the unknown acid by calculating its ioni1ation constant, !a.

1.4 Apparatus and +aterials

The materials and apparatus used in this experiment are2

• A '34 ml beaker

• 4.&M sodium hydroxide solution

• '34 ml of distilled water

• &4 ml of unknown acid solution

• $H electrode

• A burrette

• &4ml volumetric pipette

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1. T!eor"

0n the titration process, a burette is used to dispense a small, uantifiable increment of

solution of known concentration (%igure &-a). A typical burette has the smallest calibration

unit of 4.&m5 (%igure &-b), therefore, volume dispense from the burette should be estimated

to the nearest 4.4&m5.

,igure 1 - a )epicts a t"pical *#+/ burette. b Indicates s+allest calibration unit0 *.1+/0 on a t"pical *

+/ burette.

There are two general methods used to determine the !a value of a weak acid2

&. Titrating the weak acid with sodium hydroxide

'. Measuring the pH of the weak acid solution

)eter+ination of t!e a value of ea acid b" titration it! aO5

A titration curve is formed by plotting the pH of the acid solution against the volume of

"a#H added as shown in %igure '. The euivalence point of the titration is achieved when all

of the weak acid (HA) has completely reacted with "a#H. #n the titration curve, the

euivalence point is read at the middle of the region where the pH increases abruptly. The

half-euivalence point for the titration is achieved when one half of the base needed to

complete to neutrali1e the acid has been added. The concentration of the acid in the solution,

6HA7 is eual to the concentration of its con8ugate base,

A¿−¿¿¿

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,igure 2 - Titration curve of ea acid titratd b" aO5

9uivalence point2

11.36−9.209.20−6.52

=11−V V −10

2.16

2.68

=11−V

V −10

2.16V – 21.60=29.48 – 2.68V

4.84V =51.08

V =10.55mL

Half −equivalence point =10.55mL

2 =5.28mL

6.00−5.285.28−4.00

=5.20− pH pH −4.88

0.72

1.28=

5.20− pH pH −4.88

0.72 pH −3.51=6.66−1.28 pH

5


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2 pH =10.17

pH =5.09

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0n this experiment, a solution whose pH is resistant to impurities is prepared

by titrating the weak acid (HA) with a strong base, "a#H.

Na A(aq)−¿

+ H 2 O( l) NaOH (aq )+ HA (aq) ↔¿ (9uation &)

The sudden change in the pH of the solution shows that the titration has reached the

euivalence point. pH in an aueous solution is related to its hydrogen ion concentration.

:ymbolically, the hydrogen ion concentration is written as 6+¿

H 3

O¿

7. pH is defined as the

negative of the logarithm of the hydrogen ion concentration.

pH ; - log 6+¿

H 3O¿

7 (9uation ')

The uantitative measure of the position of the euilibrium is called the euilibrium constant.

%or an acid dissociation, the euilibrium constant is called the acid dissociation constant,

or K a

. 0t is defined as follows2

(9uation +)

A(aq)−¿

H 3O(aq)+¿ +¿

HA(aq )+ H 2 O(l)↔¿

+¿ A¿

−¿¿¿¿

H 3

O¿¿

¿ K a=¿

0n the case of a weak acid, the actual value of !a is a small number, indicating that only a

small fraction of the acid is dissociated into ions at any given time.


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(9uation =)

A¿−¿¿

[ H A]=¿

, assume this concentration as X , so that

A¿−¿¿

[ HA ]=¿

+¿ A

¿−¿¿¿¿+¿

H 3O¿ X

¿+¿

H 3O¿

¿ H 3 O

¿¿¿

K a

=¿

Taking the negative sign of the logarithm on each side, we can derive 2

(9uation 3)

+¿ H 3 O

¿

−log K a=−log ¿

p K a= pH

¿5.09

%or euation 3, it indicates that the p K a for the acid is eual to the pH of the solution at

the half euivalence point. The K a of the acid is determined from the p K a value as

follows 2

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K a=10− p K a

¿10−5.09

¿8.128 x10−6

%or the second method, initial pH of the weak acid is used to determine its K a .

pH of unknown acid ; =.4

H ¿+¿¿

¿

A(aq)−¿

H 3O(aq)+¿ +¿

HA(aq )+ H 2 O(l)↔¿

0nitial (M) 4.& - 4 4

1.0

x10

−4

>

1.0 x10−4

9uilibrium (M) 4.&- 1.0 x10−4

- 1.0 x10−4

1.0 x10−4

+¿ A¿−¿¿¿¿

H 3

O¿¿

¿ K a=¿

¿ [1.0 x10−4 ] [1.0 x 10−4]

[1.0

]

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¿1.0 x 10−8

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1.% Procedure

& &4ml of unknown acid was taken into the beaker.

' '44ml of "a#H solution was taken too into the different beaker.

+ The distilled water were added to the unknown acid so that the pH electrode can sink

into the bottom of the beaker.

= The "a#H solution then were poured into the burette to proceed the titration.

3 The initial pH of the unknown acid was recorded in the table.

* 'ml of "a#H solution was titrated by using the burette into the beaker that containing

unknown acid and distilled water.

/ The $h of the solution was taken.

:teps * and / were repeated until the pH reading is approximately constant.

The titration were repeated for second trial.

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1.$ 'esult and Calculation

6olu+e of aO5 7+l p5

Titration 1 Titration 2

4.44 =.&/ =.&'&.44 =.=* =.==

'.44 =./& =.*/

+.44 =. =./

=.44 3.&& 3.4*

3.44 3.'' 3.'3

*.44 3.+/ 3.='

/.44 3.3 3.*'

.44 3.& 3.

.44 *.& *.+=

&4.44 &4.= &4.'

&4.34 &4./* &4.*/&&.44 &4.* &4.

&&.34 &&.4/ &&.4+

&'.44 &&.&/ &&.&'

&'.34 &&.'= &&.'&

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8et!od 1 7Titration of ea acid it! sodiu+ !"dro9ide solution

Titration 1

0 2 4 6 8 10 12 14

0

2

4

6

8

10

12

pH

Volume of NaOH (ml)

pH of the solution

9uivalence $oint

10.50+6.182 ; .+= pH

%rom graph at pH .+3, the volume of "a#H is .34 m5

%rom calculation the volume of "a#H is

10.50−8.348.34−6.18 ;

10.00−V V −9.00

? ; .34 m5

Therefore at euivalence point pH .+=, the volume of "a#H is .34 m5

Half- 9uivalence $oint

9.50mL

2 ; =./3 m5

5.22− x x−5.11 ;

5.00−4.754.75

−4.00

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x ; 3.&

The pH at half-euivalence point is 3.& and the volume of "a#H is =./3 m5

Half- 9uivalence $oint is $ka


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10.29−8.328.32−6.34 ;

V −9.00¿

10.00−V ¿

? ; .34 m5

Half- 9uivalence $oint

9.50mL

2 ; =./3 m5

5.00−4.754.75−4.00 ;

5.25− y y−5.06

y ; 3.'4

The pH at half-euivalence point is 3.'4 and the volume of "a#H is =./3 m5.


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8et!od 2 7fro+ initial concentration and p5 of ea acid

Titration 1

• 0nitial pH ; =.&/

•


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Titration 2

• 0nitial pH ; =.&'

•


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[7.586 x 10−5][¿❑7.586 x10−5]

[0.1] K a=¿

K a=5.755 x10−8

Average ioni1ation constant of weak acid, !a

Ka= 4.571 x10−8

+5.755 x10−8

2

Ka=5.163 x10−8

p!a ; -log !a

p!a ; -log 5.163 x 10

−8

p!a ; /.'/

1.& )iscussion

The aim of the experiment is to find out the unknown acid by calculating its !a value

by titration with the standardi1ed sodium hydroxide solution. The unknown acid based on

theory is !H$.

The following of apparatus were set up to do the titration.

Burette '44ml beaker pH electrode Cetort stand

A titration curve is formed by plotting the pH of the acid solution against the volume

of "a#H added. The euivalence point of the titration is achieved when all of the weak acid

(HA) has completely reacted with "a#H. #n the titration curve, the euivalence point is read

at the middle of the region where the pH increases abruptly.

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The half-euivalence point for the titration is achieved when one half of the base

needed to complete to neutrali1e the acid has been added. The concentration of the acid in the

solution, 6HA7 is eual to the concentration of its con8ugate base,

A¿−¿¿¿

The sudden change in the pH of the solution shows that the titration has reached the

euivalence point. pH in an aueous solution is related to its hydrogen ion concentration.

:ymbolically, the hydrogen ion concentration is written as 6+¿

H 3

O¿

7. pH is defined as the

negative of the logarithm of the hydrogen ion concentration.

0n the case of a weak acid, the actual value of !a is a small number, indicating that

only a small fraction of the acid is dissociated into ions at any given time.


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The following ethics and precautions below should be practised 2

• eliminated the bubble in the burette to get the accurate reading.

• Cinse your glassware with distilled water. Dry (at least the outside) with a paper

towel. Ceturn it to the drawer from whence it came.• Avoid too fast a titration rate could cause you to overshoot

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1.( Conclusion

Based on the experimental data collected, the K a of unknown acid is from method & is

6.383 x10−6

. %rom method ', the Ka of unkown acid is5.163 x10−8

The unknown acid used in this experiment is $otassium Hydrogen $hthalate (!H$) ,

!H


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1.1* 'eco++endations

A few recommendations is suggested on the techniues employed during the experiment.

%irst, in order to prevent the bubbles from appear in the burette, rinse the burette with a few

3ml of aliuots of base before filing.

Also, for future studies, it is recommended to use a different approach. Ge need to take '

times reading to avoid the parallax error. And it is to also get the euivalence point more

accurate. Make sure to place your eyes perpendicularly to get the accurate readings.

%or measuring the distilled water, more accurate apparatus should be use. "ot to forget for

safety precautions, wear your safety goggles. 0f you spill any acid or base on your clothing,

wash it off immediately with running water.

%or the waste disposal, pour the excess "a#H and the buffer solutions down the drain,

followed by plenty of running water. 9mpty the titrated solutions and the remainder of your

unknown acid or base solution in an inorganic waste containers provided in the fume hood.

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1.11 'eferences

• T. Mettler, (n.d.), Titration :olution, retrieved from www.mt.comtitration-solutions

• Iuniata, (n.d.), Determination of the !a of a Geak Acid, retrieved from

www. services.8uniata.edu...labs ...procedural'4labs Determi nation-of-ka-of-a-

weak-acid

• A.

determination of ka of unknown acid

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