Experiment 20: Preparation of Standard Sodium Hydroxide Solution and Titration of Acetic Acid in Vin

OBJECTIVES

 

• To prepare a standard sodium hydroxide solution.
• To determine the molar concentration and mass/mass percent concentration of acetic acid in an unknown vinegar solution.
• To gain proficiency in the laboratory technique of titration.

 

DISCUSSION

 

In this experiment, we will neutralize an acidic solution of vinegar using a basic solution of sodium hydroxide. We determine the amount of sodium hydroxide necessary by performing a titration using a buret. When the acid is completely neutralized by the base, the titration stops.

 

This is called the endpoint in the titration and is signaled when an indicator changes color. At the endpoint in the titration, a single drop of base is sufficient to bring about a permanent color change. Figure 20.1 illustrates a typical titration.

 

230 Experiment 20 Copyright © 2013 Pearson Education

 

Preparation of a Standard Sodium Hydroxide Solution

 

We begin by diluting 6 M NaOH with water. Since diluting NaOH provides only an approximate concentration and it is necessary to know the concentration of NaOH precisely, we will prepare a standard solution by titration.

 

PROCEDURE

 

A. Preparation of Sodium Hydroxide Solution
1. Half-fill a 1000-mL Florence flask with ~500 mL of distilled water. Measure ~15 mL of6M NaOH into a graduated cylinder and pour the NaOH into the Florence flask.

 

Stopper the flask, and carefully swirl to mix the solution.

 

2. Condition a buret with NaOH solution from the Florence flask. Use a small funnel and half-fill the buret with NaOH. Allow some solution to pass through the buret tip into a 150-mL beaker, and empty the remainder into the sink.
3. Close the stopcock, and fill the buret with NaOH solution from the Florence flask. Note: Carefully add NaOH solution to the funnel so as to not overfill the buret.
4. Label the 125-mL Erlenmeyer flasks #1, #2, and #3. Precisely weigh ~0.5 g of KHP into each of the flasks. Add ~25 mL of distilled water to each flask, and heat as necessary to dissolve the KHP crystals.
Note: If a digital electronic balance is available, the Instructor may direct students to tare weigh the KHP samples.
5. Titrate three KHP samples as follows:
• Drain NaOH through the tip of the buret to clear any air bubbles.
• Position Erlenmeyer flask #1 under the buret as shown in Figure 20.1.
• Record the initial buret reading (± 0.05 mL).
• Add a drop of indicator to the flask.
• Titrate with NaOH to a permanent endpoint while slowly swirling the flask.
• Record the final buret reading (± 0.05 mL).
6. Refill the buret with NaOH solution, record the initial buret reading, add a drop of indicator to flask #2, titrate the KHP sample, and record the final buret reading.
7. Refill the buret with NaOH solution, record the initial buret reading, add a drop of indicator to flask #3, titrate the KHP sample, and record the final buret reading.
8. Calculate the molarity of the NaOH solution for each trial. Record the average molarity of NaOH in the Data Table of Procedure B.
B. Titration of Acetic Acid in Vinegar
1. Obtain ~50 mL of vinegar solution in a dry 100-mL beaker. Record the unknown number in the Data Table.
2. Condition a pipet with unknown vinegar solution, and transfer a 10.0-mL sample into each 125-mL flask (see Appendix E). Add ~25 mL of distilled water into each flask.

 

Note: It is not necessary to use dry flasks.

 

3. Titrate three vinegar samples as follows:
• Position Erlenmeyer flask #1 under the buret.
• Record the initial buret reading (± 0.05 mL).
• Add a drop of indicator to the flask.
• Titrate with NaOH to a permanent endpoint while slowly swirling the flask.
• Record the final buret reading (± 0.05 mL).
4. Refill the buret with NaOH solution, record the initial buret reading, add a drop of indicator to flask #2, titrate the vinegar sample, and record the final buret reading.
5. Refill the buret with NaOH solution, record the initial buret reading, add a drop of indicator to flask #3, titrate the vinegar sample, and record the final buret reading.
6. Calculate the molarity of acetic acid, HC2H3O2, in the unknown vinegar solution.
7. Convert the molarity of HC2H3O2 (60.06 g/mol) to mass/mass percent concentration. Assume the density is 1.01 g/mL for the unknown vinegar solution.