Determination of Acetic Acid - JCCC Single Sign-on

CHEMISTRY

Determination of Acetic Acid Concentration in Vinegar Using Titration

Investigation Manual

CHEMISTRY


DETERMINATION OF ACETIC ACID IN VINEGAR

Overview The goal of this experiment is to accurately determine the concentration of acetic acid in commercial vinegar. In Activity 1, the percent of acetic acid in the included vinegar sample is determined. In Activity 2, a different sample of vinegar will be used and titrated. Activity 3 is a paper activity to calculate the pKa of the acetic acid based on sample pH data that is provided.

Outcomes• Calculate the molarity and percent by mass of a solute in

solution.• Determine the concentration of an acid using titration.• Understand the role of pH indicators in titrations.• Calculate the pKa of a weak acid based on experimental data.• Calculate the percent by mass of a component in a solution.

Time RequirementsPreparation .......................................................................5 minutesActivity 1: Titration of Vinegar with Sodium Hydroxide ...................................................................45 minutesActivity 2: Titration of Unknown Vinegar Sample with Sodium Hydroxide ......................................................30 minutesActivity 3: Calculations of pKa from Sample Lab Data .............................................................................15 minutes

2 Carolina Distance Learning

Table of Contents

2 Overview 2 Outcomes2 Time Requirements3 Background5 Materials6 Safety6 Preparation7 Activity 19 Activity 2 10 Disposal and Cleanup10 Activity 3 11 Resources

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KeyPersonal protective equipment (PPE)

goggles gloves apronfollow link to video

photograph results and

submit

stopwatch required

warning corrosion flammable toxic environment health hazard

KeyPersonal protective equipment (PPE)

goggles gloves apronfollow link to video

photograph results and

submit

stopwatch required

warning corrosion flammable toxic environment health hazard

BackgroundAqueous solutions fall into one of three categories—acidic, basic, or neutral—depending on the number of hydrogen ions (H+) and hydroxide ions (OH–) present. Water molecules constantly dissociate into H+ and OH–, which then come back together to re-form water molecules. It should be noted that the H+ ion does not actually exist as such in an aqueous solution. Rather, H+ ions will associate with adjacent oxygen atoms, forming hydronium ions (H3O+).

However, for the sake of convenience in our discussion of pH, we will refer to H+ rather than H3O+.

The pH scale ranges from 0 to 14. The neutral center of the scale, 7, represents the amount of H+ per liter of water at 25 °C, which is equal to 1 × 10–7 moles. Solutions with a pH of less than 7 possess more H+ and fewer OH– ions, and are considered acidic. Solutions with a pH greater than 7 possess less than 1 × 10–7 H+ (and therefore more OH–) and are considered basic, or alkaline. A solution with an equal number of H+ and OH– is considered neutral. Note that, because pH values are based upon logarithms of the base 10, each pH value varies from the next by a factor of 10 in terms of strength. For example, a solution with a pH of 5 is 10 times more acidic than one with a pH of 6.

Logarithms and pHIt is useful to be able to determine the pH of an aqueous solution if the amount of H+ is known; likewise, sometimes one knows the pH of the solution but needs to deduce the amount of H+. Brackets in an equation, such as [H+] below, refer to the concentration.

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To determine pH when [H+] is known, use the following formula:

pH = –log[H+]

For example: Determine the pH of an aqueous solution in which [H+] = 1 × 10–3 M.

pH = –log[H+] = –log(1 × 10–3) = 3

To determine [H+] when the pH of a solution is known, use the same formula. The notation “inv log” refers to the inverse log.

For example: An aqueous solution has a pH of 5.5. Determine [H+] in M (moles/liter).

pH = –log[H+] = 5.5

log[H+] = –5.5

[H+] = inv log (–5.5) = 3.2 × 10–6 M

Vinegar, a widely used preservative, cleaning product, and cooking ingredient, is a solution of acetic acid (CH CO H3 2 ). Different types of vinegar are produced through the fermentation of a wide variety of fruits and grains. Distilled white vinegar is produced by the fermentation of alcohol derived mainly from corn.

A titration is the precise addition of a solution of known concentration and composition, called the titrant, to a solution of unknown concentration or composition, called the analyte. In this acid-base titration, the titrant, sodium hydroxide, is delivered through a syringe in order to measure precisely (±0.1 mL) the volume of base added. In many traditional acid-base titrations, the titrant is added using a burette, a piece of glassware designed

continued on next page


Background continuedfor adding known volumes of a solution to another container. Acetic acid, a weak acid, can be neutralized with a strong base, such as sodium hydroxide, to form a salt and water. The concentration of acetic acid, the analyte, in vinegar can be determined through an acid-base titration.

CH3CO2H + NaOH → NaCH3CO2 + H2O

At the equivalence point of a titration, the moles of acid added is exactly equal to the moles of base in the solution before the titration started. The equivalence point is achieved when sufficient titrant has been added to react with all of the analyte. The end point is the experimental estimate of the equivalence point, and is typically identified by a color change in an indicator solution. In a titration, there is a noticeable change in pH near the end point. A change in color of an indicator added to the analyte at the start of the titration is observed when the end point has been reached. The molarity and volume of the titrant are known, as is the volume of the analyte. To determine the concentration of the analyte, first determine the number of moles of titrant that were needed to reach the end point. Next, use the balanced chemical equation to calculate the moles of analyte present. Finally, use the volume of the analyte to find the concentration of the analyte.

The pH range of the indicator used for a given titration must include the pH change of the end point. Acid-base indicators are weak organic acids that change color depending on the pH of the solution. The pH range of an indicator is the range of pH values over which the color change occurs when the indicator converts

between its acidic and basic forms. This range depends on the strength of the indicator’s acid form and, therefore, varies among indicators. Phenolphthalein is the indicator used in this experiment. Phenolphthalein is colorless in its acidic form and pink in its basic form. The pKa of this indicator is 9, and it has a pH range of 8–10, which is appropriate for the titration to be performed.

It is important to remember that acids and bases will dissociate in aqueous solutions. The acid dissociation constant (Ka) is the equilibrium constant of the dissociation reaction of an acid in aqueous solution, representing a dynamic balance between the acid in its intact and dissociated states. The pKa is a simplified representation of Ka.

The pKa of the acetic acid can easily be calculated using the data obtained in this lab and the Henderson-Hasselbalch equation:

pH = pKa + log[A–]/[HA]

pKa = –log Ka

The Ka, the acid dissociation constant, is the equilibrium constant for the dissociation reaction:

HA ↔︎ H+ + A–

where an acid (HA) dissociates to form its conjugate base (A–) and a hydrogen ion (H+). Because the value of Ka can vary over several orders of magnitude, it is convenient to use the –log of this number, the pKa, for reference. In the pKa equation, [A–] is the molar concentration of the anion and [HA] is the molar concentration of




the acid. For practical purposes, the pKa should be equal to the pH at the midpoint of a titration. The midpoint is the volume that is halfway to the end point of a titration. At the midpoint, [HA] = [A-] and the pH measured at the midpoint can be used to experimentally determine the pKa.

MaterialsIncluded in the equipment kit:

Erlenmeyer flask, 25 mL

Plastic cup, 1.25 oz

Graduated cylinder, 10 mL

Electronic balance


Syringe, 10 mL

Needed from the materials kit:

Needed from the chemical kit #1:

Vinegar, acetic acid solution



Activity 1 will be performed using the vinegar that comes with the kit. Activity 2 will be performed using a different vinegar sample to compare concentrations. Find some malt, rice, or other type of vinegar for the second titration. Some restaurants have small packets of malt vinegar as a condiment.

Materials continuedNeeded from the chemical kit #2:

SafetyWear your safety goggles, chemical apron, and gloves at all times while conducting this investigation.

Read all the instructions for this laboratory activity before beginning. Follow the instruc-tions closely and observe established laboratory safety practices, including the use of appro-priate personal protective equipment (PPE) as described in the Safety and Activity sections.

Phenolphthalein is an alcohol based solution. Keep chemical away from any heat or flame sources.

Acetic acid and sodium hydroxide are corrosive. In the event of contact with skin or eyes, the affected area should be immediately rinsed with water for 15 minutes.

Do not eat, drink, or chew gum while performing this activity. Wash your hands with soap and water before and after performing the activity. Clean up the work area with soap and water after completing the investigation. Keep pets and children away from lab materials and equipment.

Preparation1. Read through the Procedure.2. Obtain all materials.3. Clean and sanitize work area.

Phenolphthalein indicator solution, 1%

Sodium hydroxide solution, 1 M

Reorder Information: A replacement kit for Determination of Acetic Acid Concentra-tion in Vinegar Using Titration, item number 580314, can be ordered from Carolina Biological Supply Company.

Call 800-334-5551 to order.

Needed, but not supplied:• Plain sheet of white paper• Vinegar sample


ACTIVITY

ACTIVITY 1

A Titration of Vinegar with Sodium Hydroxide1. Place the 10-mL graduated cylinder on the

electronic balance.2. Tare the balance.3. Dispense ~ 5 mL of vinegar into the

graduated cylinder with a plastic pipet.4. Record the exact mass of vinegar in Data

Table 1.5. Transfer the vinegar into the clean 25-mL

Erlenmeyer flask.6. Add 2 drops of phenolphthalein to

the vinegar. Take photograph of flask as the start point of the titration.

7. Transfer approximately 10 mL of 1 M sodium hydroxide (NaOH) to the small cup. This solution will be used to fill the syringe.

8. Fill the syringe with 1 M NaOH solution.Try to minimize the amount of air in the syringe.

9. Record the initial volume of NaOH solution in the syringe to the nearest 0.1 mL in Data Table 1 for Vinegar Sample 1. Read the volume of the syringe at the edge of the plunger.

10. Place the flask containing the 5.0 mL of vinegar on the white sheet of paper. The white background will make the color change more noticeable.

11. Add NaOH from the syringe in 0.5-mL increments, swirling the flask between each addition until a color change persists for a few seconds after the addition of the NaOH.

12. Add NaOH dropwise from the syringe, with constant swirling, until a single drop of NaOH causes a color change that persists for a few seconds after the addition. This indicates that the end point has been reached.

13. Once the end point has been reached, record the final volume of NaOH in the syringe in Data Table 1 for Vinegar Sample 1. Take photograph of flask as the end point of the titration.

14. Calculate the total volume of NaOH that was required to reach the end point and record the data in Data Table 1.

15. Dispose of the titrated vinegar sample in the sink.

16. Rinse the 25-mL flask three times with water.17. Repeat steps 1–16 for Vinegar columns 2

and 3 on the Data Table 1.18. Calculate the concentration of acetic acid

in your vinegar sample based on your experimental data The molar mass of acetic acid is 60.05 g/mol. See the following example:Example: A sample of 10.0 grams of HCI solution of unknown concentration is titrated to the end point with 6.2 mL of a 1.0 M NaOH sample. Calculate the concentration of hydrochloric acid.

To gain more control of single drops from the syringe, move the plunger by holding it where it enters the syringe instead of pressing the end of the plunger.


1. Calculate the volume of HCl solution. Volume = Mass / Density Volume = 10.00 g / 1.00 g/mL = 10.00 mL

2. Determine the moles of sodium hydroxide used to reach the end point. Moles of NaOH = 1.0 M × 6.2 mL × 1 L / 1,000 mL = 0.0062 moles NaOH

3. Determine the moles of hydrochloric acid in the sample. Moles of NaOH = Moles of HCl at the equivalence point Moles of NaOH = 0.0062 moles = Moles of HCl

4. Calculate the molarity of hydrochloric acid. 0.0062 moles / 0.010 L = 0.62 M HCl

5. Calculate the mass of hydrochloric acid in the initial sample. Mass of HCl = Moles of HCl / Molar Mass of HCl Mass of HCl = 0.0062 mol × 36.46 g/mol = 0.23 g

6. Calculate the percent mass of hydrochloric acid in the initial sample. Percent Mass = (Mass of Solute / Total mass) × 100 Percent Mass = (Mass of HCl / Mass of solution) × 100 Percent Mass = (0.23 g / 10.00 g) x 100 = 2.3 %

Data Table 1: Titration of Vinegar with Sodium Hydroxide

Name of Vinegar Sample Vinegar Sample 1 Vinegar Sample 2 Vinegar Sample 3

Mass of VinegarVolume of Vinegar Density = 1.005 g/mL

Initial NaOH Volume in Syringe

Final NaOH Volume in Syringe

Volume of NaOH Delivered

Moles of NaOH Delivered

Moles of Acetic Acid in Sample

Mass of Acetic Acid in SampleCalculated Molarity of Acetic Acid in the Sampled VinegarCalculated Percent Mass of Acetic Acid in Sampled VinegarAverage Calculated Percent Mass of Acetic Acid in Sampled Vinegar

ACTIVITY


ACTIVITY 1 continued

ACTIVITY 2

A Titration of an Unknown Vinegar Sample with Sodium HydroxideActivity 2 is an inquiry-based activity to demon-strate the learned technique of determining the acetic acid concentration in vinegar to a vinegar sample from your home. Each type of vinegar has a slightly different concentration of acetic acid. This activity requires the selec-

tion of a vinegar product from your home, a friend’s home, or a local restaurant. Only about a teaspoon of vinegar is needed. Attempt to find a different type of vinegar such as cider, malt, or balsamic vinegar. The vinegar included in this kit is white vinegar. Different brands of vinegar may also contain different amounts of acetic acid. Use the same procedure and Data Table 2 to determine the acetic acid content in your vinegar sample.

Data Table 2: Titration of Second Vinegar Sample with Sodium Hydroxide

Name of Vinegar Sample Vinegar Sample 1 Vinegar Sample 2 Vinegar Sample 3

Mass of Vinegar

Volume of Vinegar Density = 1.005 g/mL

Initial NaOH Volume in Syringe

Final NaOH Volume in Syringe

Volume of NaOH Delivered

Moles of NaOH Delivered

Moles of Acetic Acid in Sample

Mass of Acetic Acid in Sample

Calculated Molarity of Acetic Acid in the Sampled Vinegar

Calculated Percent Mass of Acetic Acid in Sampled Vinegar

Average Calculated Percent Mass of Acetic Acid in Sampled Vinegar


solve a mathematical problem. The pKa of acetic acid is 4.75, and the volume of vinegar is 21.05 mL.

1. Review the lab data found in Data Table 3. This is from a titration of a vinegar sample using a pH meter.

2. Determine the volume of 0.1 M NaOH dispensed during the titration. Record the answer.

3. Calculate the molarity of the acetic acid in the vinegar samples. Record the answers.

4. Calculate the pKa of acetic acid from the midpoint data using the Henderson-Hasselbach equations in the Background section.

Disposal and Cleanup1. Dispose of titrated samples in the sink.2. Clean and dry the flask and return it to the

equipment kit.3. Return unused chemicals to the chemical kit.4. Sanitize work space.

ACTIVITY 3

A Calculations of pKa from Sample Lab DataThe following activity is intended as an extension activity and requires the application of concepts from the Background and Activities 1 and 2 to

Data Table 3: Titration of Vinegar with Sodium Hydroxide

Vinegar Sample 1 Vinegar Sample 2 Vinegar Sample 3

Initial pH Reading pH = 3.211 pH = 3.102 pH = 3.105 Initial Buret Reading 2.89 mL 3.00 mL 1.61 mL Midpoint pH Reading pH = 4.895 pH = 4.687 Midpoint Buret Reading 12.92 mL 9.50 mL Final pH Reading pH = 8.238 pH = 8.531 pH = 8.934 Final Buret Reading 20.12 mL 20.31 mL 19.00 mL Volume of NaOH dispensedCalculated Molarity of Acetic Acid in the Vinegar SampleVolume NaOH dispensed at Midpoint ReadingMoles of NaOH Consumed at Midpoint Reading [A–]Moles of Acetic Acid Remaining at Midpoint [HA]Calculated pKa


ACTIVITY


5. Find the average calculated molarity, percent by mass concentration, and pKa for acetic acid based on the provided data.

ResourcesSetting up and Performing a Titrationhttp://www.carolina.com/teacher-resources/Video/setting-up-and-performing-a-titration-video/tr11191.tr

Titration Calculationshttp://www.carolina.com/teacher-resources/Video/titration-calculations-video/tr11190.tr


ACTIVITY 3 continued

Average calculated molarity of acetic acid:

_________________________________________

Average acetic acid concentration expressed in percent by mass:_________________________________________

Average value of pKa of acetic acid based on provided experimental data:_________________________________________

Percent error for pKa of acetic acid: _________________________________________

http://www.carolina.com/teacher-resources/Video/setting-up-and-performing-a-titration-video/tr11191.tr

http://www.carolina.com/teacher-resources/Video/titration-calculations-video/tr11190.tr







CHEMISTRY Determination of Acetic Acid Concentration in Vinegar Using Titration


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