Determination of the ka of a weak acid

Final Report

Exercise 1

CHEM 182 DL1 Determination of Ka for a Weak Acid

1. Using the data collected in Data Table 2, create a graph of “Drops of NaOH added” vs “pH” where “drops of NaOH added” is on the x-axis and “pH” is on the y-axis. Upload an image of the graph into Graph 1.

We use the pH indicator strips to calculate the amount of unknown acid in the receiving flask by measuring the specific amount of base it takes to neutralize the acid. There are two major ways to know when the solution has been fully neutralized. The first uses a pH meter in the beaker by slowly adding base until the pH reads exactly 7 (neutral). The next method uses a specific indicator. An indicator is an acid or base whose conjugate acid or base has a color difference from the original compound. The color changes when the solution contains a 1:1 ratio mixture of the differently colored forms of the indicator. The pH equals the p K a of the indicator at the endpoint of the indicator. Since the pH of the solution is known and the volume of titrate is added, we can then deduce how much base was needed to neutralize the unknown sample.

2. pH indicator strips work by changing color in the presence of solutions with varied pH values. T hinking about your procedure steps and results in Part 1, why do you think the pH of the unknown weak acid was not determined with pH indicator strips until Part 2?

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Data Table 1: Determination of Equivalence Point

My results were close with a 5% Ka error. This error could have come from the pH key, which may not have been 100 percent read precisely. Also, incorrect measuring of solutions when preparing the experiment could have resulted in error.

3. Discuss possible causes of error in the experimental procedure. Why do you think there was a percent of error in the pK and K values of the unknown weak acid, in comparison to the values presented in Table 2?

a a

A pH meter has to be specifically calibrate correctly, so if this was calibrated incorrectly, it could also result in error, however I think that there would be a less of a percent error because a meter instrument is more accurate than strips and gives an exact numerical measurement.

4. Do you think using a pH meter instead of pH indicator strips would have created a larger or smaller percent error? Explain your answer.

Phenolphthalein is a great indicator for determining the equivalence point. Is a suitable indicator for strong acid or a strong base. It is also a good indicator for a weak acid or strong base therefore it works well in this experiment.

5. Why was phenolphthalein a good indicator to use for determining the equivalence point between the unknown weak acid and strong base?

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Data Table 2: Titration Curve Values

Trial 1

Trial 1

Volume of Unknown Weak Acid

Total Number of Drops Required to Reach Equivalence Point

Average Number of Drops

Trial 1

Trial 1

Volume of Unknown Weak Acid

Total Number of Drops Required to Reach Equivalence Point

Average Number of Drops

Trial 2

Trial 1

Volume of Unknown Weak Acid

Total Number of Drops Required to Reach Equivalence Point

Average Number of Drops

4ml

114 required drops

109

4ml

104 required drops

Drops NaOH Added pH Value Trial 1

Half-Equivalent Point

Equivalent Point

0

10

54

109

3

2.5

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20

30

40

50

60

70

80

90

100

110

120

Drops NaOH Added pH Value Trial 2

Half-Equivalent Point

Equivalent Point

0

10

20

30

40

50

3

3.5

4

4.5

5

6

6.5

7

7.5

8

11

54

109

1

2.5

3

3.5

4.5

5

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60

70

80

90

100

110

120

Drops NaOH Added pH Value (Average)

Half-Equivalent Point

Equivalent Point

0

10

20

30

40

50

60

70

80

90

5.5

6

6.5

7.25

7.5

8

11.5

5.0

1

2.5

3.25

4

4.25

4.75

5.5

6

6.5

7.5

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Panel 1: Equivalent Point Number of Drops

Panel 2: Half-Equivalent Point Number of Drops

Data Table 3: Determination of Unknown

Graph 1: Drops NaOH vs pH

100

110

120

7.75

8

12

The Equivalent Point= 93 Drops

The Half- equivalent point= 46 Drops

pK of Unknown Weak Acid:

K of Unknown Weak Acid:

Unknown Weak Acid Identity:

% Error pK :

% Error K :

a

a

a