identifying of unknown monoprotic acid

8/10/2019 identifying of unknown monoprotic acid

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4.0 THEORY

Our second experiment is identifying an unknown weak acid based on the Ka value obtained

from titration method and half volume method. We used these two methods in order to

identify the unknown acid and at the end of the experiment we had determined which method

is more accurate.

Theoretically, strength of and acid depends on its ability to lose proton (H +). Weak acids

usually will partially dissociated and release some of its hydrogen ion in the solution. Hence

its ability to donate proton is much lower than strong acid as it partially dissociated in water.

Therefore, these kind of acids have higher pKa than strong acids as strong acids completely

dissociated in water and release all the hydrogen atoms.

Ka, acid dissociation constant is defined asthe quantitative measure of the strength of and acid

in a solution. pKa, is equal to log10 Ka.Ka is the equilibrium constant for a chemical

reaction known as dissociation in the context of acid-base reactions. Symbolically,

equilibrium can be written as below :

HA (aq) + H2O (l) H3O+ (aq) + A- (aq)

Under such equilibrium conditions, the total concentrations of each species remain constant

even though the species in solution are constantly dissociating and recombining.

Monoprotic acid is an acid that donates only one proton or hydrogen atom per molecule to an

aqueous solution and will only have one equivalence point. As for polyprotic acid, literally it

corresponds to the transfer of more than one proton.

The ionization constant of weak monoprotic acid can be calculated as below :

Ka = [H+] [A-]

[HA]


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As we all know that the greater the value of Ka the higher the formation of H +is favoured so

that the pH of the solution become lower. Theoretically the Ka of weak acid varies between

1.810-16and 55.5. Acids that are lesser than that value are said to be weaker acids than water

itself.

There is one other way that can be used to identify an unknown acid by determining its

percent of dissociation which is symbolized as (alpha) and which it ranges from 0%

100%.Percent dissociation can be defined as :

= [A-]

[A-] + [HA]

As is not constant so it does not depends on the value of [HA]. Generally will increase as

[HA] decreases hence acids become stronger as they are diluted. Each proton will have Ka as

it said to be polyprotic. In other words percentage of ionization refers to the proportion of

neutral particles, such as those in a gas or aqueous solution which are later ionized into

charged particles.

The acid ionization constant, Ka can be determined from the titration curve. The pH values at

particular volume of sodium hydroxide being added can be obtained from the titration curve

so that the pKa values can be obtained from the plotted titration curve. As the data obtained,

two titration curves of pH versus volume of base added were plotted then the equivalence

point was obtained.

The pH values obtained from the titration curve can be used to calculate the acid ionization

constant hence the average value can be determined. Plus, the volume of base titrated at the at

the equivalence point is used to calculate the concentration of the unknown monoprotic acid.

The average is determined as well and the average acid ionization constant later will be used

to identify the unknown monoprotic acid.


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Apparatus and Materials

1. pH meter and electrode

2. 250 ml beaker

3.

40 ml of unknown acid

4. Distilled water

5.

Burette

6. Retort stand

7. Magnetic stir plate

8.

Magnetic stirrer

9. Graduated measuring cylinder

10.

NaOH solution

11.Erlenmeyer flask

Burette

Beaker

Unknown

acid

Magnetic stirrer

Magnetic plate

pH electrode

Retort stand


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Half volume method

1.

The burette was cleaned, rinsed and filled with NaOH solution.

2.

An unknown acid of 40 ml was measured. 100 ml of distilled water measured and

poured into a 250 ml of Erlenmeyer flask. The unknown acid was dissolved in

distilled water.

3. The solution we prepared in step 2 was divided into two parts using graduated

measuring cylinder.

4.

The solution prepared then titrated with NaOH until pH 12 was reached.

5. The titrated solution then was mixed with the solution which has not been titrated. The

value of Ka was once again determined from the [H+] value which was linked with the

pH of the solution.


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7.0 RESULTS

7.1 Titration of the unknown monoprotic acid with 0.1 M sodium hydroxide, NaOH.

Burette Reading Titration 1 Titration 2 Titration 3

0 3.76 3.66 3.711 3.90 3.81 3.86

2 4.06 3.96 4.01

3 4.21 4.08 4.11

4 4.32 4.19 4.20

5 4.42 4.29 4.40

6 4.52 4.40 4.50

7 4.62 4.49 4.55

8 4.70 4.56 4.60

9 4.79 4.65 4.71

10 4.87 4.72 4.8011 4.96 4.80 4.96

12 5.05 4.88 5.00

13 5.13 4.96 5.12

14 5.25 5.04 5.20

15 5.37 5.15 5.27

16 5.52 5.25 5.50

17 5.70 5.39 5.60

18 6.01 5.54 5.96

19 6.83 5.73 6.45

20 10.23 6.02 7.13

21 10.88 7.10 10.3622 11.02 10.01 10.80

23 11.11 10.52 11.10

24 11.19 10.76 11.19

25 11.24 10.89 11.23

26 11.29 11.01 11.25

27 11.36 11.08 11.32

28 11.40 11.14 11.41

29 11.42 11.19 11.45

30 11.45 11.23 11.47

31 11.47 11.27 11.4932 11.49 11.30 11.52

33 11.51 11.33 11.55

34 11.53 11.35 11.56

35 11.55 11.38 11.62

36 11.56 11.43 11.63

37 11.60 11.55 11.66

38 11.64 11.61 11.68

39 11.69 11.65 11.70

40 11.74 11.72 11.73

41 11.77 11.78 11.80

42 11.81 11.83 11.82

43 11.87 11.90 11.89


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44 11.92 11.92 11.90

45 11.96 11.95 11.93

46 12.01 11.99 12.02

Table 7.1

7.2 Half method titration of the unknown monoprotic acid with 0.1 M sodium

hydroxide, NaOH.

Burette Reading Titration 1 Titration 2 Titration 3

0 3.50 3.51 3.51

1 3.88 3.90 3.89

2 4.21 4.17 4.20

3 4.44 4.42 4.41

4 4.65 4.61 4.63

5 4.87 4.79 4.85

6 5.23 4.99 5.35

7 5.47 5.23 5.39

8 6.52 5.70 6.11

9 10.31 7.50 8.91

10 10.66 10.41 10.56

11 10.86 10.76 10.81

12 10.97 10.91 10.94

13 11.07 11.03 11.05

14 11.14 11.11 11.12

15 11.20 11.17 11.1916 11.26 11.23 11.24

17 11.31 11.28 11.30

18 11.35 11.32 11.33

19 11.39 11.36 11.37

20 11.42 11.39 11.40

21 11.45 11.43 11.44

22 11.49 11.46 11.47

23 11.52 11.51 11.50

24 11.66 11.63 11.64

25 11.79 11.75 11.7626 11.87 11.86 11.85

27 11.93 11.91 11.92

28 11.96 11.97 11.98

29 12.00 12.01 12.00


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7.3 Graph for the titration of the unknown monoprotic acid with 0.1 M sodium

hydroxide, NaOH.

7.3.1 Titration 1

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46

pHmonoproticacid

Volume NaOH, ml

1) pH monoprotic acid Vs. Volume NaOH, ml


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7.3.2 Titration 2

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46

pHm

onoproticacid

Volume NaOH, ml



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7.3.3 Titration 3

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46

pHmonoproticacid

Volume NaOH, ml



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7.4 Graph for the half method titration of the unknown monoprotic acid with 0.1 M

sodium hydroxide, NaOH.

7.4.1 Titration 1

7.4.2 Titration 2

7.4.3 Titration 3


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8.0 CALCULATIONS

8.1 The titration of the unknown monoprotic acid with 0.1 M sodium hydroxide,

NaOH

8.1.1 Titration 1

By applying Henderson Hesselbalch Equation,

pKa = -log Ka + log

, since within the buffer region, the concentration of base reacted

with the concentration of acid was equal, so, log

= 0

Thus,

pKa = -log Ka

4.85 = -log Ka

Ka = -antilog 4.85

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46

pHmonoproticacid

Volume NaOH, ml


19.50 ml

pKa = 4.85

9.75 ml


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= 1.41 x 10-5

8.1.2 Titration 2

Applying the Henderson Hesselbalch Equation

pKa = -log Ka

4.80 = -log Ka

Ka = -antilog 4.8

= 1.58 x 10-5

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46

pHmonoproticacid

Volume NaOH, ml


21.40 ml

pKa = 4.80

10.70 ml


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8.1.3 Titration 3


pKa = -log Ka

4.80 = -log Ka

Ka = -antilog 4.8

= 1.58 x 10-5

8.2 The half method titration of the unknown monoprotic acid with 0.1 M sodium

hydroxide, NaOH

8.2.1 Titration 1


0

2

4

6

8

10

12

14

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46

pHmono

proticacid

Volume NaOH, ml


20.50 ml

10.25 ml

pKa = 4.80


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pKa = -log Ka

4.65 = -log Ka

Ka = -antilog 4.65

= 2.24 x 10-5

8.2.2 Titration 2


pKa = -log Ka

4.70 = -log Ka

Ka = -antilog 4.70

= 2.00 x 10-5

8.2.3 Titration 3


pKa = -log Ka

4.70 = -log Ka

Ka = -antilog 4.70

= 2.00 x 10-5


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8.3 Titration table for the titration of the unknown monoprotic acid with 0.1 M

sodium hydroxide, NaOH

Titration

No.

Volume Of

NaOH at

equivalence

point, ml

pH at

equivalence

point

Volume of

NaOH

halfway to

equivalence

point, ml

pKa value Ka value

1 19.50 8.40 9.75 4.85 1.41 x 10-5

2 21.40 8.30 10.70 4.80 1.58 x 10-5

3 20.50 8.82 10.25 4.80 1.58 x 10-5

Average Ka 1.52 x 10-5

8.4 Titration table for half method titration of the unknown monoprotic acid with 0.1

M sodium hydroxide, NaOH

Titration

No.

Volume Of

NaOH at

equivalence

point, ml

pH at

equivalence

point

Volume of

NaOH

halfway to

equivalence

point, ml

pKa value Ka value

1 8.25 8.20 4.125 4.65 2.24 x 10-5

2 9.25 8.20 4.625 4.70 2.00 x 10-5

3 8.75 8.25 4.375 4.70 2.00 x 10-5

Average Ka 2.08 x 10-5


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9.0 Discussion

In this experiment, we have been assigned to carry out two types of method that focused on

the identifying of unknown monoprotic acid; titration curve method and half method. In the

expression, Ka is the acid dissociation constant. Strong acids typically dissociate completely,

and therefore would have a Ka value of greater than 1. Weak acids have Ka values much

smaller than 1 (typically less than 10-4).

For the first method, the titration of as much as 50 ml of NaOH was titrated into 20 ml of

unknown weak acid had been conducted until up to three titration. All of those titrations were

left to be until the pH value reached around 12. In the interest of identifying the unknown

weak acid that we worked on, we need to know the Ka value of the acid from each of the

three titration curves (D.C. Harris, 2004).

What we knew about the titration curve was that there was a buffer region that located right

along the way till to the equivalence point. Buffer region is a region that we added just

enough bases in the acidic solution that we have some conjugate base in the solution, and we

also have some acidic solution (D.C. Harris, 2004). The Ka value could be obtained from this

region.

In the simplest way, the Ka value from the titration curve can be obtained from the value of

pKa by applying the Henderson Hesselbalch Equation (A. Marie, 2014). pKa value in each

titration curve was exactly equal to the pH halfway to the equivalence point.

The pKa value had noticed to be 4.85, for titration 1, while titration 2, and 3 had recognized

the same pKa value of 4.80. As it went into the calculations, the Ka value of the acid had

accordingly became 1.41 x 10-5for titration 1 and the following titrations had seen an equal

Ka value of 1.58 x 10-5. The total average of these Kas was 1.52 x 10 -5. This value had not

been exactly equal but quite close to that of propanoic acid, C3H6O2(1.3 x 10-5) (Table of

Ka, 2010).

Meanwhile, from the half volume method, the pKa value of 4.65 for the titration 1was 4.65,

and for both of titration 2 and 3 the value was recorded to be 4.70. After using the Henderson

Hesselbalch Equation, the Ka value had been showing the corresponding value match the


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recognizable unknown acid. The Ka for first titration was 2.24 x 10 -5 while the second and the

third titration had seen an equal Ka value of 2.00 x 10-5. The average Ka for three titrations of

half volume method, was 2.08 x 10-5. So, from our sources table of Ka, the close value Ka of

unknown acid from our experiment of half volume method was falling closer toward the Ka

value of Crotonic acid (HC4H5O2).

From the both method, we identified the unknown acid that used in this experiment but we

got different weak acid. For first method the unknown acid identified as propanoic acid while

for the second method as a crotonic acid. The difference value pKa of first method and

second method just around 0.14. May errors preparation occur during the experiment caused a

difference in determining the unknown acid.

The deviation of the experimentally obtained average Ka value from any other close

Ka values from the Ka table might due to several errors that had incidentally associated

during the completion of this experiment. Mainly, it might due to the fact that the parallax

error had occurred. In a way, the volume on the burette was misread. This can be a parallax

mistake when the observer looking at the volumes on the incorrect angle. In addition, we did

not know the exact concentration of the NaOH we used in this experiment, as we had not

standardized the basic solution in prior to the execution of experiment. So, it could be that

error might have resulted from the uses of incorrect concentration of NaOH. Besides, it could

be that during the titration, there was several times where the solution had been transferred in

excess volumes by incorrectly leveling the scale on the burette.

10.0 Conclusion

The titration curves in this experiment have shown a consistent shape pattern for all

three titration of the both method with each of these curves have recognized slight differencesin pH value in the equivalence point. In the interest of identifying the monoprotic acid, the Ka

value of the acid was determined from the pHs that correspond to the volume of bases

halfway to the equivalence point from the titration curves. The pKa value had noticed to be

4.85, for titration 1, while titration 2, and 3 had recognized the same pKa value of 4.80.

Meanwhile, the Ka value of the acid had accordingly recognized to be 1.41 x 10 -5for titration

1 and the following titrations had seen an equal Ka value of 1.58 x 10 -5. The average value of

Ka was seen to be close to the value of Ka of propanoic acid, which was 1.30 x 10 -5from the


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theoretical Ka table. While the pKa of half method slightly differ from first method that

caused difference result in identification of monoprotic weak acid.

REFERENCES

A. Marrie. (2014).About.com Chemistry:Henderson Hesselbalch Equation and Example.

Retreived fromhttp://chemistry.about.com/od/acidsbase1/a/hendersonhasselbalch.htm

D.C. Harris. (2004).Determination of the Identify of an Unknown Weak Acid [Online PDF

Handout]. Retreived fromhttp://apbrwww5.apsu.edu/robertsonr/chem1110-

20/044%20Unknown%20Acid%20Ka%20MM.pdf

Table of Ka Values for Common Monoprotic Acids. (2015). Retrived from

http://www.bpc.edu/mathscience/chemistry/table_of_monoprotic_acids.html

P.Austin.(2012).Determining the Identity of an Unknown Weak Acid . State University

Department of Chemistry. Retrieved from

http://www.apsu.edu/sites/apsu.edu/files/chemistry/F12

Anonymous.Ka of an Unknown Acid. Retrieved April,22 2014 from

http://www.ccchemistry.us/ch%20111%20experiment%2015.pdf
http://chemistry.about.com/od/acidsbase1/a/hendersonhasselbalch.htmhttp://chemistry.about.com/od/acidsbase1/a/hendersonhasselbalch.htmhttp://chemistry.about.com/od/acidsbase1/a/hendersonhasselbalch.htmhttp://apbrwww5.apsu.edu/robertsonr/chem1110-20/044%20Unknown%20Acid%20Ka%20MM.pdfhttp://apbrwww5.apsu.edu/robertsonr/chem1110-20/044%20Unknown%20Acid%20Ka%20MM.pdfhttp://apbrwww5.apsu.edu/robertsonr/chem1110-20/044%20Unknown%20Acid%20Ka%20MM.pdfhttp://apbrwww5.apsu.edu/robertsonr/chem1110-20/044%20Unknown%20Acid%20Ka%20MM.pdfhttp://www.bpc.edu/mathscience/chemistry/table_of_monoprotic_acids.htmlhttp://www.bpc.edu/mathscience/chemistry/table_of_monoprotic_acids.htmlhttp://www.apsu.edu/sites/apsu.edu/files/chemistry/F12http://www.apsu.edu/sites/apsu.edu/files/chemistry/F12http://www.ccchemistry.us/ch%20111%20experiment%2015.pdfhttp://www.ccchemistry.us/ch%20111%20experiment%2015.pdfhttp://www.ccchemistry.us/ch%20111%20experiment%2015.pdfhttp://www.apsu.edu/sites/apsu.edu/files/chemistry/F12http://www.bpc.edu/mathscience/chemistry/table_of_monoprotic_acids.htmlhttp://apbrwww5.apsu.edu/robertsonr/chem1110-20/044%20Unknown%20Acid%20Ka%20MM.pdfhttp://apbrwww5.apsu.edu/robertsonr/chem1110-20/044%20Unknown%20Acid%20Ka%20MM.pdfhttp://chemistry.about.com/od/acidsbase1/a/hendersonhasselbalch.htm


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Anonymous.Identification of an Unknown Weak Acid. Retrieved April,22 2014 from

http://web.utk.edu/~kcook/319S02/exp5m.pdf
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