Dissociation Constant of Acetic Acid DCP CE

The pH of various mixtures and find the dissociation constant of acetic acid
Chemistry (HL)
Research Question:
To determine the pH of various mixtures of sodium acetate and acetic acid in an aqueous solution and hence to find the dissociation constant of the acetic acid.

Background:
The mixture of sodium acetate and acetic acid in aqueous solution is a buffer solution. Buffer solutions are the solutions which resist a sudden change in the pH due to addition of small amounts of strong acid or base. An acidic buffer is a mixture of a weak acid and its salt with a strong base. A basic buffer is a mixture of a weak base and its salt with strong acid. For example,
CH3COOH + CH3COONa is an acidic buffer and NH4CI + NH2OH is a basic buffer. The pH of an acidic buffer HA + BA is given by the Henderson equation which is pH = pKa + log
According to the equation, the pH of an acidic buffer depends on the ratio because pKa is constant. The pH of any solution can be accurately determined by pH metric method and if the ratio is known, the dissociation constant can be calculated.

Apparatus:

Material
Quantity
Uncertainties pH Meter
1
± 0.02 pH
Glass Electrode
1
-
Volumetric Flasks (50cm3)
5
-
Beaker (1000cm3)
2
-
Beaker (50cm3)
1
-
Water Bath
1
-
Measuring Cylinder (50 cm3)
1
± 0.5 cm3
Measuring Cylinder (10 cm3)
1
± 0.05 cm3
Acetic Acid (0.1M)
-
-
Sodium Acetate (0.1M)
-
-
Potassium Hydrogen phthalate solution (0.05M)
-
-

Procedure:
Preparation of Solutions:
Preparing the buffer solutions using 0.1 M acetic acid and 0.1 M sodium acetate solutions:

Calibration of pH meter using the Potassium Hydrogen Phthalate solution.
Wash the pH electrode with distilled water
Take 50 cm3 of 0.05 M Potassium Hydrogen Phthalate solution in a beaker
Dip the glass electrode into the solution, make sure that the bulb of the glass electrode is completely dipped into the solution
Adjust the course and fine standardization knob so that pH meter read the pH = 4
After this adjustment do not disturb the instrument throughout the investigation.

Measuring the pH of the buffer solutions:
1. First mix the solutions and name it as ‘A’
2. In a small beaker take the buffer solution A. Dip the calomel and glass electrode after washing and ensure that the bulb of the glass electrode is immersed completely in the solution
3. Adjust the temperature selector switch at room temperature value
4. Put the pH-range selector switch at 0 - 7 pH-range
5. Note down the pH of the solution A.
6. Repeat the steps (1 – 5) for taking five trials of solution A.
7. Repeat the steps (1 – 5) for taking five trials of solution (B to L).

Risk Assessment:
While performing this investigation, use of pH meter is done, while pH meter instrument consists of the glass electrode which is a very sensitive part in the instrument. So, care should be taken while dipping the glass electrode in the buffer solutions and while cleaning to take readings for other trials.
It is necessary to wear laboratory goggles, apron and gloves and mask while performing this investigation to avoid contact from the above harmful chemicals with eyes and skin.

Data Collection:

Qualitative Data:
There will be no quantitative data as there was no color change experienced from the naked eye.

Raw Data Table (Observation Table):
Table 1 – Raw Data Table showing the different pH values taken for the following buffer solutions

Buffer Solution
Acetic Acid (0.1 M) cm3
Sodium Acetate (0.1 M) cm3 pH of the both mixtures

Trial 1
Trial 2
Trial 3
Trial 4
Trial 5
A
47.5
2.5
3.26
3.24
3.23
3.21
3.20
B
45
5.0
3.60
3.62
3.65
3.63
3.61
C
40
10
3.91
3.93
3.89
3.92
3.95
D
35
15
4.16
4.14
4.17
4.15
4.13
E
30
20
4.39
4.37
4.39
4.37
4.34
F
25
25
4.61
4.63
4.67
4.64
4.62
G
20
30
4.78
4.79
4.76
4.74
4.76
H
15
35
4.97
4.95
4.93
4.96
4.98
I
10
40
5.18
5.17
5.19
5.16
5.15
J
7.5
42.5
5.39
5.36
5.35
5.37
5.32
K
5.0
45
5.56
5.58
5.55
5.57
5.54
L
2.5
47.5
5.72
5.73
5.75
5.74
5.71

Data Processing:
Formula to calculate the average pH value of the buffer solutions:

= Total pH of buffer solutions ÷ No. of Trials

= Trial 1 + Trial 2 + Trial 3 + Trial 4 + Trial 5 ÷ 5 (No. of Trials)

Sample Calculation for buffer solution A = 3.26 + 3.24 + 3.23 + 3.21 + 3.20 ÷ 5 = 3.23

Table 2 – Data Table is showing the average pH values of the buffer solution and the standard deviation for them.

Buffer Solution
Acetic Acid (0.1 M) cm3
Sodium Acetate (0.1 M) cm3
Average pH of the buffer solutions
Standard Deviation
A
47.5
2.5
3.23
0.02
B
45
5.0
3.62
0.02
C
40
10
3.92
0.02
D
35
15
4.15
0.02
E
30
20
4.37
0.02
F
25
25
4.63
0.02
G
20
30
4.77
0.02
H
15
35
4.96
0.02
I
10
40
5.17
0.02
J
7.5
42.5
5.36
0.03
K
5.0
45
5.56
0.02
L
2.5
47.5
5.73
0.02

Formula to calculate the concentration of acid in buffer solution:

In acetic acid:

[Acid] =

[Acid] = = 0.095 mol dm-3

In Sodium Acetate:

[Salt] =

[Salt] = = 0.005 mol dm-3

Table 3 – Data Table is showing the concentration of acid in Acetic acid and sodium acetate and the log.

Buffer Solution
Acetic Acid (0.1 M) cm3
Sodium Acetate (0.1 M) cm3
Concentration of Acid in Acetic Acid [Acid] (mol dm-3)
Concentration of Acid in Sodium Acetate [Salt] (mol dm-3) log Average pH of the buffer solutions
A
47.5
2.5
0.095
0.005
-1.28
3.23
B
45
5.0
0.09
0.01
-0.95
3.62
C
40
10
0.08
0.02
-0.60
3.92
D
35
15
0.07
0.03
-0.37
4.15
E
30
20
0.06
0.04
-0.18
4.37
F
25
25
0.05
0.05
0.00
4.63
G
20
30
0.04
0.06
0.18
4.77
H
15
35
0.03
0.07
0.37
4.96
I
10
40
0.02
0.08
0.60
5.17
J
7.5
42.5
0.015
0.085
0.75
5.36
K
5.0
45
0.01
0.09
0.95
5.56
L
2.5
47.5
0.005
0.095
1.28
5.73

Data Representation:

Graph 1:
The below graph is showing the relationship between the Average pH of the buffer solutions (mixtures) and the log.

Fig.1

The above graph (Fig.1) shows the relationship between the average pH of the buffer solutions and the log. From the graph it is clearly seen that the values of the relationship between the two axes are increasing respectively. If the average pH of the buffer solutions is increasing, the log is also increasing. The graph is showing a linear trend in the values. It is having an inclined rising, which tells that the graph is going toward the positive trend. For example, for buffer solution ‘A’, the average pH value, 3.23 where the log value for the same is, -1.28, then as the pH increases the log also increases, like for the buffer solution ‘B’, the average pH value, 3.62 where the log value for the same is, -0.95, which clearly signifies that as the average pH of the buffer solutions is increasing, the log is also increasing leading to increasing positive trend line. The intercept of Y – axis gives the value of pKa, where from the graph it can be seen that the value of pKa is 4.63 and its antilog will be calculated further. The linear trend line covers all the twelve points in the graph, which shows that the average pH readings that have been obtained through this investigation are accurate.

Calculating the Dissociation Constant of Acetic Acid (ka) from graph:

Note: The value of the pH at the intercept of Y – axis was taken from graph:

The intercept of Y – axis gives the value of pKa = 4.63

Conclusion:

In the end, from the raw data table it could be perceived that as the quantity of Acetic acid in the buffer solution is decreasing, the quantity of Sodium acetate is increasing and the pH of both the mixtures is rising. For example, buffer solution ‘A’, the quantity of acetic acid is 47.5 cm3 and the quantity of the Sodium Acetate is 2.5 cm3, so the average pH value which is seen in the data table 2 is 3.23, where for buffer solution ‘B’, the quantity of acetic acid is 45 cm3 and the quantity of the Sodium Acetate is 5.0 cm3, so the average pH value for this buffer solution is 3.62. Now, it is seen that as the quantity of acetic acid is decreasing and the quantity of sodium acetate is increasing, therefore the pH for that particular buffer solution is increasing.

The main aim behind this investigation was to calculate the dissociation constant of the acetic acid by using the method pH metric. To calculate the dissociation constant, first the concentration of acid in both the solutions, Acetic acid and Sodium acetate are first calculated. The final result is as follows, the literature value of the dissociation constant of acetic acid is 1.8 × 10-5 and the value which has been obtained from this investigation is 2.3 × 10-5. The dissociation constant obtained from the investigation is not too close and not too far to the literature value, so this shows that the investigation performed successfully. However, the dissociation constant value is not too close which means that the investigation had some errors. This gives the value of log, where in some cases the value is also negative and zero. From the data table of concentration of acid in Acetic acid and sodium acetate and the log, it can be seen that the value of log is increasing from negative to positive. From the graph, it can be seen that as the as the average pH of the buffer solutions is increasing, the log is also increasing leading to increasing positive trend line and hence the investigation is proven.

Errors
Improvements
While preparing the buffer solution, some solution might have stayed in the apparatus which would cause an error.
For the improvement, the
The apparatus such as conical flask, beakers and measuring cylinder may not have been properly cleaned before using it for the investigation is not cleaned properly which leads to the presence of systematic error.
This error can be reduced by using different sterilized apparatus to measure the quantity/volume of the solutions, so they are not mixed with other solutions.