Materials Included In Kit

Additional Materials Required

Prelab Preparation

Potassium hydrogen phthalate, KHC₈H₄O₄: Dry the solid for at least two hours in an oven at 110 °C. Store the dry solid in a desiccator. It must be cool when its mass is measured.

Unknown Concentrations of Vinegar

1. Label a 1-L beaker “Vinegar Unknown Concentration A.”
2. Pour the acetic acid solution, 1 M, into the beaker.
3. Label a 1-L beaker “Vinegar Unknown Concentration B.”
4. Pour the acetic acid solution, 1.6 M, into the beaker.
5. Label a 1-L beaker “Vinegar Unknown Concentration C.”
6. Pour the acetic acid solution, 0.833 M, into the beaker.

Safety Precautions

The sodium hydroxide solution is moderately toxic by ingestion and skin absorption. It is corrosive to body tissues and causes severe eye burns. Avoid all body contact with bases and acids. Acetic acid solution is corrosive to skin and eyes and slightly toxic by ingestion and inhalation. Phenolphthalein is an alcohol-based solution and is flammable. It is moderately toxic by ingestion and a possible carcinogen. Keep it away from flames and other ignition sources. Avoid contact of all chemicals with eyes and skin and remind students to wash hands thoroughly with soap and water before leaving the laboratory. Wear chemical splash goggles and chemical-resistant gloves and 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 regulations that may apply, before proceeding. The solid acid may be treated according to Flinn Suggested Disposal Method #24a. Excess sodium hydroxide solutions may be handled according to Flinn Suggested Disposal Method #10. Excess acetic acid may be neutralized according to Flinn Suggested Disposal Method #24b. The titrated solutions may be rinsed down the drain according to Flinn Suggested Disposal Method #26b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Each group can analyze two of the three unknown acetic acid concentrations. All parts of this laboratory activity can reasonably be completed in two 50-minute class periods. The Prelab Questions may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• Before having students start the lab, demonstrate how to use a buret and how to titrate a solution. Show students how to fill a buret and also an example of when an endpoint is reached (the solution should be a very light pink). The Flinn Scientific Laboratory Techniques Guide (Catalog No. AP6248) provides thumbnail illustrations of these and 14 other common laboratory techniques.
• Remind students to slide the desiccator lid off.
• Instead of swirling the Erlenmeyer flasks, students can use a magnetic stir bar and stirrer.
• If solutions will be used the following day, cover solutions with Parafilm^® to avoid evaporation.

Teacher Tips

• Depending on class time, students can analyze one or two unknown acetic acid solutions. Enough materials are provided for each group to analyze two of the unknown acetic acid samples, if desired.
• Once all groups have calculated the concentration of their unknown acid, data can be compiled from all groups and an average for each unknown can be calculated. For example, have each group list their final concentration on the board for the class to average.
• Additional dilute acids can be titrated, as an extension. For example, a 0.06 M citric acid solution.
• Titration requires patience and detailed quantitative analysis. Quantitative analysis represents a nearly invisible application of chemistry in our daily lives. To illustrate the importance of quantitative analysis, ask students how they would feel if they could not trust that the water they drink or the medicines they take had been tested to assure quality and safety.
• Total Acidity—Titration of Fruit Juices (Flinn Catalog No. AP6690) is a student laboratory kit that can be used for an extension of titrations.

Answers to Prelab Questions

1. What is the molar mass of acetic acid? Show your calculations.

   HC₂H₃O₂
   2(12.01) + 4(1.01) + 2(16.00) = 60.06 g/mol

2. If 22.3 mL of 1.0 M NaOH is needed to titrate 35.0 mL of acetic acid, what is the molarity of the acetic acid solution?

   M_aV_a = M_bV_b
   M_a(35.0 mL) = (1.0M)(22.3 mL)
   M_a = 0.637 M

3. Using the answer from Question 2, calculate the percent acetic acid in the sample.

   Given 1 liter of solution, there are 0.637 moles of acetic acid.
   0.637 moles acetic acid × (60.06 g acetic acid/1mol acetic acid) = 38.25822 g acetic acid
   % acetic acid = (38.25822g/1000g) x 100 = 3.83%

Sample Data

Standardization Data Table

Molarity NaOH (average) ___1.01___ M

Unknown Concentration Data Table
Unknown Letter ___A___ Concentration of Unknown (average) ___1.01___ M
Concentration of Unknown (average) ___6.07___ %

Unknown Concentration Data Table (optional)
Unknown Letter ___B___ Concentration of Unknown (average) ___1.61___ M
Concentration of Unknown (average) ___9.67___ %

Answers to Questions

1. From the standardization data, calculate the molarity of the sodium hydroxide solution for each trial. Average the values and enter the average in the Standardization Data Table.
   {14134_Answers_Equation_9}
2. From the unknown concentration data, calculate the molarity of the vinegar solution for each trial. Average the values and enter the average in the Unknown Concentration Data Table.

   Unknown A

   Trial 1

   (1.01 M)(0.02060 L) = M_a(0.0205 L)
   M_a = 1.0149 M

   Trial 2

   (1.01 M)(0.01970 L) = M_a(0.0200 L)
   M_a = 0.9949 M

   Trial 3

   (1.01 M)(0.01950 L) = M_a(0.0195 L)
   M_a = 1.010 M

   Molarity (Average): (1.0149 + 0.9949 + 1.010)/3 = 1.01 M

   Unknown B

   Trial 1

   (1.01 M)(0.03130L) = M_a(0.0195 L)
   M_a = 1.62 M

   Trial 2

   (1.01 M)(0.03310 L) = M_a(0.0210 L)
   M_a = 1.59 M

   Trial 3

   (1.01 M)(0.03250 L) = M_a(0.0205 L)
   M_a = 1.60 M

   Molarity (Average): (1.62 + 1.59 + 1.60)/3 = 1.60 M

3. From the unknown concentration data, calculate the percentage of vinegar in the sample from the average molarity. Enter the average percent in the Unknown Concentration Data Table.

   Unknown A

   Molarity = 1.01 M. Given 1 liter of solution, there are 1.01 moles of acetic acid.
   1.01 moles acetic acid x (60.06 g acetic acid/1mol acetic acid) = 60.6505 g acetic acid
   % acetic acid = 60.6505 g/1000g × 100 = 6.07%

   Unknown B

   Molarity = 1.60 M. Given 1 liter of solution, there are 1.60 moles of acetic acid.
   1.60 moles acetic acid x (60.06 g acetic acid/1mol acetic acid) = 96.096g acetic acid
   % acetic acid = 96.096 g/1000g x 100 = 9.61%

   Unknown C

   Molarity = 0.833 M. Given 1 liter of solution, there are 1.01 moles of acetic acid.
   0.833 moles acetic acid x (60.06 g acetic acid/1mol acetic acid) = 50.02998 g acetic acid
   % acetic acid = 50.02998 g/1000g x 100 = 5.00%

4. Why must the KHP samples be dried? If they are not dried, how would the results change (high or low)?

   If the KHP is moist when weighed, a higher mass of KHP would be measured, and the calculated molarity of the NaOH would be too high.

5. Why must NaOH be standardized? Why can’t an exact solution of NaOH be prepared?

   Because sodium hydroxide pellets rapidly absorb water from the air, the precise mass of NaOH cannot be measured on an analytical balance.

Introduction

Manufacturers are constantly performing quality control on products to make sure standards are met. In this lab, a vinegar factory needs your help! Several barrels of vinegar were mixed up and mislabeled. The factory needs you to determine the percentage of vinegar in the barrels and correctly label each container.

Concepts

• Titration
• Acids and bases
• Concentration

Background

Vinegar is a dilute aqueous solution of acetic acid produced by the fermentation of apple juice (cider vinegar), grapes (wine vinegar), or barley malt (malt vinegar). Federal regulations require that vinegar contains at least 4% acetic acid by mass. If the amount of acetic acid is less than 4%, the acidity level may not be high enough to prevent the growth of bacteria in pickled or canned foods.

In this experiment the percentage of acetic acid present in a white vinegar solution will be determined by titrating it with a known solution of sodium hydroxide. Acetic acid, a weak acid, reacts with sodium hydroxide, a strong base, via the neutralization reaction shown in Equation 1.

In this titration, the exact volume of sodium hydroxide needed to react completely with a measured volume of vinegar will be recorded. When all of the acid has been neutralized, the number of moles of acid (moles_a) must be equal to the number of moles of base (moles_b), as shown in Equation 2. Molarity is defined as moles of solute per liter of solution (a unit of volume), as shown in Equation 3. Rearranging the units in the definition of molarity provides an equation for the number of moles of solute (Equation 4). Combining Equations 2 and 4 gives Equation 5, which can be used to calculate the molarity of acetic acid in vinegar (M_a) based on titration with a standard base solution (M_b). Since volume appears on both sides of Equation 5, any units may be substituted in Equation 5, as long as they are identical for both the acid and base. Thus, volume may be measured in liters, milliliters, or even drops from a pipet.

The mass of the acid per liter of solution can then be calculated using Equations 6 and 7. or 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.

Experiment Overview

The purpose of this experiment is to titrate and identify the concentration of various vinegar samples. First, a sodium hydroxide solution will be standardized and then the standardized solution will be used in the titration.

Materials

Prelab Questions

1. What is the molar mass of acetic acid? Show your calculations below.
2. If 22.3 mL of 1.0 M NaOH is needed to titrate 35.0 mL of acetic acid, what is the molarity of the acetic acid solution?
3. Using the answer from Question 2, calculate the percent acetic acid in the sample.

Safety Precautions

All the acids and bases used in this lab are irritating to eyes, skin and other body tissues. Phenolphthalein is an alcohol-based solution and is flammable. It is moderately toxic by ingestion and a possible carcinogen. Keep away from flames and other ignition sources. Avoid contact of all chemicals with eyes and skin and wash hands thoroughly with soap and water before leaving the laboratory. Wear chemical splash goggles and chemical-resistant gloves and apron.

Procedure

Part A. Preparing a 1.0 M NaOH solution.

1. Measure 83.3 mL of 3.0 M sodium hydroxide solution in a graduated cylinder.
2. Place the 3.0 M sodium hydroxide solution into a 250-mL beaker.
3. Dilute the sodium hydroxide to 250 mL by slowly adding 166.7 mL of distilled or deionized water.

Part B. Standardization of a Sodium Hydroxide Solution

4. Obtain a sample of potassium hydrogen phthalate (KHP) that has been previously dried in an oven and stored in a desiccator.
5. On an analytical balance, accurately weigh approximately 1.0 g of KHP in a previously tared weighing dish. Record the mass of the KHP in the Standardization Data Table.
6. Transfer the KHP into an Erlenmeyer flask—pour the solid through a funnel into the flask. Use water from a wash bottle to rinse all of the remaining solid in the weighing dish or in the funnel into the flask as well.
7. Add about 100 mL of distilled water to the flask and swirl until all the KHP is dissolved.
8. Clean a 50-mL buret, then rinse it with three small portions (about 7 mL each) of the 1.0 M NaOH solution.
9. Fill the buret to above the zero mark with the 1.0 M NaOH solution.
10. Open the buret stopcock to allow any air bubbles to escape from the tip. Close the stopcock when the liquid level is between the 0- and 10-mL marks.
11. Measure the precise volume of the solution in the buret and record this value in the Standardization Data Table as the “initial volume.” Note: Volumes are read from the top down in a buret. Always read from the bottom of the meniscus, remembering to include the appropriate number of significant figures (see Figure 1).
    {14134_Procedure_Figure_1}
12. Position the buret over the Erlenmeyer flask so that the tip of the buret is within the flask but at least 2 cm above the liquid surface.
13. Add three drops of phenolphthalein solution to the KHP solution in the flask.
14. Place a white piece of paper under the Erlenmeyer flask to help view the color change.
15. Begin the titration by adding 1.0 mL of the 1.0 M NaOH solution to the Erlenmeyer flask, then closing the buret stopcock and swirling the flask.
16. Reduce the incremental volumes of NaOH solution to 0.5 mL until the pink color starts to persist. Reduce the rate of addition of NaOH solution to drop by drop until the pink color persists for 15 seconds. Remember to constantly swirl the flask and to rinse the walls of the flask with distilled water before the endpoint is reached.
17. Measure the volume of 1.0 M NaOH remaining in the buret, estimating to the nearest 0.01 mL. Record this value as the “final volume” in the Standardization Data Table.
18. Repeat the standardization titration two more times. Rinse the Erlenmeyer flask thoroughly between trials with deionized water.

Part C. Determining the Molarity of a Sample of Vinegar with an Unknown Concentration

19. Obtain 60.0 mL of vinegar with an unknown concentration. Record the unknown letter in the Unknown Concentration Data Table.
20. Measure 20.0 mL of the vinegar into a clean Erlenmeyer flask. Record the volume in the Unknown Concentration Data Table.
21. Add 20 mL of distilled water to the flask.
22. Add 3 drops of phenolphthalein in the flask.
23. Fill the buret with 1.0 M sodium hydroxide to between the 0- to 10-mL marks.
24. Measure the precise volume of the solution in the buret and record this value in the Unknown Concentration Data Table as the “initial volume.” Note: Volumes are read from the top down in a buret. Always read from the bottom of the meniscus, remembering to include the appropriate number of significant figures (see Figure 1).
25. Place a white piece of paper under the Erlenmeyer flask to help view the color change.
26. Slowly titrate the vinegar with the 1.0 M sodium hydroxide solution.
27. When the solution in the Erlenmeyer flask stays a light pink, stop the titration and record the final volume of 1.0 M NaOH in the buret in the Unknown Concentration Data Table.
28. Repeat steps 20–27 two more times.
29. (Optional) Repeat steps 19–28 with another unknown vinegar sample.
30. Dispose of the solutions and any solid as directed by your instructor.

Student Worksheet PDF

14134_Student1.pdf