How to find pka of unknown acid

Cailin Winokur

Professor Stage

Chemistry II

March 18, 2018

Lab 6: Determination of Ka for a Weak Acid

Procedure: To begin the experiment, carefully cut the tip of the phenolphthalein pipet and add exactly 2 drops of phenolphthalein solution to the unknown weak acid in the beaker. Then swirl the beaker to completely mix the unknown weak acid and phenolphthalein solution. Remove the cap of the Sodium Hydroxide and fill an empty pipet full of the solution. Add NaOH from the pipet into the beaker containing the 4 mL of unknown weak acid one drop at a time, swirling and observing the solution in the beaker after each drop until the color changes to a pale-pink color for at least 5 seconds. Count and record each drop of NaOH added to the beaker, so that you will know exactly how many drops of NaOH were required to cause the solution to change from clear and colorless to pale-pink in color. Next, carefully pour the solution down the sink drain and use the dish soap, tap water, and paper towels to wash and dry the beaker. Repeat, observe, and record. Next, add exactly 10 drops of NaOH from the pipet into the beaker containing the 4 mL of unknown. Gather a pH indicator strip from the kit and place it into the solution in the beaker so that all 4 indicator squares come into contact with the weak acid. Accurately read the pH indicator, record, and repeat. After finishing the experiment, calculate the percent error based on the results that was gathered throughout the experiment.

Exercise 1:

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.

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2. pH indicator strips work by changing color in the presence of solutions with varied pH values. Thinking 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?

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.

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

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.

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.

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.

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

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.

Data Table 1: Determination of Equivalence Point

Trial 1

Trail 2

Volume of Unknown weak Acid

4ml

4ml

Total number of Drops required to reach equivalence point

114 required drops

104 required drops

Average Number of Drops

Average: 109

Data Table 2: Titration Curve Values

Drops NaOH added

pH Value Trial 1

pH Value Trail 2

pH Value Average

Half-Equivalent Point

54

54

5.0

Equivalent Point

109

109

0

3

1

1

10

2.5

2.5

2.5

20

3

3

3.25

30

3.5

3.5

4

40

4

4.5

4.25

50

4.5

5

4.75

60

5

5.5

5.5

70

6

6

6

80

6.5

6.5

6.5

90

7

7.25

7.5

100

7.5

7.5

7.75

110

8

8

8

120

11

11.5

12

Panel 1: Equivalent Point Number of Drops

The Equivalent Point= 93 Drops

Panel 2: Half- Equivalent Point Number of Drops

The Half- equivalent point= 46 Drops

Data Table 3: Determination of Unknown

pKa of Unknown Weak acid

5.0

Ka of Unknown Weak Acid

.00001

Unkown Weak Acid Identity

Acetic Acid

% Error pka

-44%

% error Ka

5%

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Conclusion:

This lab demonstrated how a titration can be used to measure the concentration of an weak or strong acid or base in solution. This experiment allowed me to practice how to calculate the molar mass of an unknown acid or base, and determine the equilibrium constant of a weak acid (Ka) or weak base (Kb). By adding NaOH, a strong base, in small increments, it was possible to determine the equivalence point of NaOH and the unknown weak acid. Then, the pH at equilibrium was used in order to find the concentration of the unknown acid at equilibrium. In addition, by using the mass of the unknown weak acid and its concentration at equilibrium, the molar mass of the acid was determined. Finally, by using the equation pKa = -log(Ka) and the half equivalence point pKa, the value of Ka for the unknown weak acid was found.. Even though, I had a 5% error, I still believe I conducted this experiment to the best of my ability. I really enjoyed this lab and would recommend people to try it.