Materials Included In Kit

Additional Materials Required

Safety Precautions

The phenolphthalein solution is a flammable liquid, a fire risk and is moderately toxic. Sodium hydroxide is a corrosive liquid, skin burns are possible, and it is very dangerous to the eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. The 1% phenolphthalein solution may be disposed of according to Flinn Scientific Disposal Method #18b. The 0.2 M solution of sodium hydroxide should be disposed of by following Flinn Scientific Disposal Method #10. The products from the reaction are neutralized and can be disposed of using Flinn Suggested Disposal Method #26b.

Teacher Tips

• Any 6-, 24- or 48-well reaction plate can be used for this experiment. Small plastic medicine cups can also be used.
• The following is a sample calculation: Using Equation 5, the molarity of acetic acid in the vinegar was found to be 0.76 M. The molar mass of acetic acid is 60 g/mole. Using Equation 7, the mass of acetic acid per liter of vinegar was found to be 45.6 g/L. Finally, the percent acetic acid was found to be 4.56% using Equation 8.
• Use only 1 drop of indicator, more than 1 drop may cause significant error. Be sure the thin stem pipets used to dispense the indicator have stems “drawn out” so that the diameter of the stems are reduced in size. This will help avoid dispensing too much indicator. The “drawn out” stems will deliver about 50 drops per mL, as compared to the standard stems which will deliver about 25 drops per mL. See the attached drawn-out, thin-stem pipet procedure.
• To improve accuracy, use a drawn-out pipet for all measurements.
• To make a solution of known percentage of acetic acid from a concentrated source use the following procedure:

  Glacial acetic acid solution contains 99.5% acetic acid (which is nearly pure). In order to make 1 L of a 1 M solution of acetic acid from the glacial, determine the number of milliliters of glacial acetic acid required for the process. This is done by dividing the molar mass of acetic acid by the density (1.06 g/mL).

  molar mass/density = 60.03 g/1.06 g/mL = 57 mL

  Therefore, add 57 mL of glacial acetic acid to 500 mL of water then dilute to one liter. This solution would theoretically be a 5.7% solution of acetic acid.

  (57 mL/1000 mL)(100) = 5.7%

Sample Data

Introduction

The purpose of this kit is to determine what percentage of acetic acid is present in white vinegar by titrating it with a solution of sodium hydroxide.

Concepts

• Ionization
• Dissociation
• Titration
• Molarity
• Calculating percentage of acetic acid

Background

Acetic acid is formed by the oxidation of alcohol which takes place in such processes as bacterial fermentation of apple cider, wine, or other fruit juices. There are a number of applications in which acetic acid is used, such as in the making of plastics, dyes, pharmaceuticals, and pickled foods. Vinegar is a natural product that contains acetic acid. The commercial varieties of vinegar contain a mass percentage of 4% to 8% acetic acid. The legal minimum amount is 4%.

When acetic acid is placed in water, ionization takes place forming hydronium ions (H₃O⁺). These ions cause the vinegar to taste sour, its desirable trait. In this experiment the percentage of acetic acid present in a white vinegar will be determined by titrating it with a known solution of sodium hydroxide.

To determine the percentage of acetic acid present in a sample of vinegar, first calculate its molarity. Molarity is the number of moles of a substance per liter of solution. Knowing the molarity will allow the calculation of the mass of acetic acid present in 1 L (1000 g) of the vinegar. This, in turn, will make it possible to calculate the percentage of acetic acid in the vinegar.

For monoprotic acids, one mole of base will neutralize one mole of acid. If mole_a is the number of moles of acid and mole_b is the number of moles of base, then mole_a equals mole_b (Equation 1) when all the acid has been neutralized.

Molarity (M) is defined as moles (m) divided by volume (V) where volume is always expressed in liters (Equation 2). Equation 2 can be rearranged to provide an equation for calculating moles (Equation 3). If equation 1 and 3 are combined, then the concentration and volume of an acid can be directly related to the concentration and volume of a base required to neutralize the acid (Equation 4). Since volume is on both sides of the equation, the units can be changed as long as they are identical for both the acid and base. For example, volume can be measured in liters, milliliters, or drops from a pipet (Equation 5). If the molarity of the base and the number of drops of acid and base required for neutralization is known, then the molarity of the acid can be determined using Equation 5.

Moles can also be determined by the mass of the material divided by its molar mass (Equation 6). Combining Equations 6 and 3 will provide a direct relationship between mass, molarity, and molar mass (Equation 7). The mass of the acid per liter of solution is calculated using Equation 7. Assuming one liter of vinegar has almost the same mass as one liter of water (1000 g), the percentage of acetic acid can be determined using Equation 8.

Materials

Safety Precautions

The phenolphthalein solution is a flammable liquid, a fire risk and is moderately toxic. Sodium hydroxide is a corrosive liquid, skin burns are possible, and it is very dangerous to the eyes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron.

Procedure

Titration Tip: The end point (neutralization) is reached when the solution remains a light pink color. The lighter the pink color, the closer it is to the true end point.

1. Position a 24-well reaction plate on top of a white piece of paper. Draw out three thin-stem pipets (see Figures 1–4 on Student PDF). Use one pipet for the vinegar, one for the phenolphthalein and one for the sodium hydroxide.
2. Place 10 drops of white vinegar into well A1. It is important to always hold the pipet at the same angle to obtain equalsized drops.
3. Use a clean pipet and add 1 drop of phenolphthalein indicator to the vinegar.
4. Titrate the sample of vinegar with a 0.2 molar solution of sodium hydroxide to reach the end point. In the data table, record the number of drops of sodium hydroxide used.
5. Repeat the process five more times and record your results for each trial. Calculate the average number of drops of NaOH used for the six trials. Record your result.
6. 6. See instructor for appropriate disposal procedures.

Student Worksheet PDF

12531_Student1.pdf