CHEMLAB 15 pgs. 542-543

PROBLEM

What are the volume percentages of acetic acid in several brands of vinegar?

OBJECTIVES

• Observe acid-base titrations of several vinegars with a standard sodium hydroxide solution.

• Calculate the volume percentages of acetic acid in the vinegars.

• Compare the acetic acid concentrations of various brands of vinegar

HYPOTHESIS

MATERIALS

24-well microplate

several brands of vinegar

labeled microtip pipets

standard NaOH solution

phenolphthalein solution

toothpicks

distilled water in a wash bottle

SAFETY PRECAUTIONS

Wear aprons and goggles. Sodium hydroxide is caustic and can damage skin and eyes. If you come into contact with any of this solution, rinse the affected area with a large volume of water and notify the teacher. Wash hands thoroughly when you complete the lab.

PROCEDURE

1. Use the data table to record your data and observations.

2. Use a microtip pipet to add ten drops of the first type of vinegar to each of the

wells—A1, B1, and C1—of the microplate. Record the brand of the first

vinegar in the data table.

3. Use a clean microtip pipet to add one drop of phenolphthalein indicator solution to each of the three wells.

4. Set the microplate on a piece of white paper.

5. Use a clean pipet to carefully add a drop of the standard NaOH solution to the

solution in well A1, and stir with a toothpick.

Pause for about 30 seconds and look down through the well for evidence of a

persistent, light pink, phenolphthalein color that indicates the endpoint of the

titration. Repeat this process with each drop until the endpoint is reached.

Record the number of drops of sodium hydroxide solution required to titrate the

vinegar to the endpoint.

6. Repeat procedure 5 with the second sample of this vinegar, which is in well B1. If the result differs by more than one drop from that of the first titration,

repeat again with the sample in well C1.

7. Repeat procedures 2 through 6 with the other brands of vinegar, using other

columns of wells of the microplate. Record your data after each titration.

ANALYZE AND CONCLUDE

1. Interpreting Data From the two closest trials, find the average number of drops of NaOH required to titrate each vinegar. Use this average number of drops and the given molarity of the NaOH to calculate the molarity of acetic acid in each of the brands of vinegar. Assuming identical volumes for drops of vinegar and drops of NaOH solution, the ratio of reacting volumes in liters is the same as the ratio of reacting volumes in drops.

2. Interpreting Data Use your results to calculate the volume percentage of acetic acid in each brand of vinegar according to the formula:

APPLY AND ASSESS

1. How could you have changed the experimental procedure in order to more accurately determine the concentrations of the vinegars?

2. If the cost and volume of each brand of vinegar are available, calculate the cost per percent of acetic acid per unit volume for each. Which is the best buy based upon this criterion?