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Calculating Acid in Lemon-Lime Soda (13 Favorites)

LAB in Interdisciplinary, Concentration, Acid Base Reactions, Molarity, Titrations, Indicators. Last updated October 24, 2019.


Summary

In this lab, students will investigate the molarity of citric acid in a clear, lemon-lime flavored soft drink through titrations with 0.10M NaOH and an indicator.

Grade Level

High school

AP Chemistry Curriculum Framework

This laboratory experiment supports the following units, topics, and learning objectives:

  • Unit 4: Chemical Reactions
    • Topic 4.2: Net Ionic Equations
      • TRA -1.B: Represent changes in matter with a balanced chemical or net ionic equation: a. For physical changes. b. For given information about the identity of the reactants and/or product. c. For ions in a given chemical reaction.
    • Topic 4.5: Stoichiometry
      • SPQ-4.A: Explain changes in the amounts of reactants and products based on the balanced reaction equation for a chemical process.
    • Topic 4.6: Introduction to Titration
      • SPQ- 4.B: Identify the equivalence point in a titration based on the amounts of the titrant and analyte, assuming the titration reaction goes to completion.
  • Unit 8: Acids and Bases
    • Topic 8.2: pH and pOH of Strong Acids and Bases
      • SAP-9.B: Calculate pH and pOH based on concentrations of all species in a solution of a strong acid or a strong base.
    • Topic 8.5: Acid-Base Titrations
      • SAP-9.E: Explain results from the titration of a mono- or polyprotic acid or base solution, in relation to the properties of the solution and its components.
    • Topic 8.7: pH and pKa
      • SAP-10.A: Explain the relationship between the predominant form of a weak acid or base in solution at a given pH and the pKa of the conjugate acid or the pKb of the conjugate base.

Objectives

By the end of this lab, students should be able to

  • describe the purpose and procedure of a titration.
  • define the terms: acid, base, titration, titrant, analyte, end point, indicator
  • perform a weak acid – strong base titration.
  • calculate the molarity of citric acid based on the results of the titration.
  • express awareness of health concerns of excessive consumption of acidic beverages.

Chemistry Topics

This lab supports students’ understanding of

  • Acid-base reactions
  • Titrations
  • pH scale
  • Indicators

Time

Teacher Preparation: 45 minutes

Lesson: Two class periods

Materials

Per Lab Group:

  • Buret, 50 mL
  • Ring stand
  • buret clamp
  • funnel
  • 0.10 M NaOH solution, 100 mL
  • Beaker, 250 mL
  • two beakers, 100 mL
  • Phenolphthalein indicator
  • Erlenmeyer flask, 125 mL
  • volumetric pipette, 25 mL
  • Pipette filler or bulb
  • Permanent marker
  • Approximate 60 mL sample of decarbonated lemon-lime flavored soft drink
  • Access to internet

Safety

  • Always wear safety goggles and lab apron when handling chemicals in the lab.
  • Students should wash their hands thoroughly before leaving the lab.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
  • When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
  • Nitrile gloves should be provided for students.
  • Do not consume lab solutions, even if they’re otherwise edible products.
  • Food in the lab should be considered a chemical not for consumption.
  • Teachers and Students should read the SDS for 0.10 M NaOH

Teacher Notes

  • This activity is best performed by students who are familiar with acids, bases, and titration techniques. For example, students should have already performed a strong acid – strong base titration and be familiar with the titration equation:

Ma x Va x #H’s=Mb x Vb x #OH’s

  • In this particular investigation students will be determining the molarity of the citric acid (solving for M a in the titration equation).

Where:
Ma = molarity of acid
Mb = molarity of base
#H’s = number of acidic hydrogens in formula
Va = volume of acid
Vb = volume of base
#OH’s = number of hydroxides in formula

  • The major acid in lemon-lime flavored soft drinks is citric acid (as opposed to many other sodas which have additional phosphoric acid). Citric acid (H3C6H5O7) is triprotic, but the Ka values of each of the protons are very similar. This results in a titration with only one clear equivalence point which occurs after the neutralization of the third (final) proton. The phenolphthalein indicator selected for this investigation works well for this titration since the equivalence point pH value is above 7.0 and near the range of the color change for phenolphthalein (which is around 9; pKa of 9.3). The molarity of the citric acid should come out somewhere near 0.1 M for most lemon-lime sodas.
  • To prepare the soft drink for use in the lab it must be decarbonated. Simply leave the soda uncapped at room temperature for at least twenty-four hours prior to use.
  • This activity could be used as an enrichment lesson showing how titrations can be applied to answering questions relevant to everyday life, and having students explore the health effects of acids in their everyday life by reading several web-based articles. It could also be used as extension of weak acid-strong base titrations.

Cross-Disciplinary Extensions

Connect to Math
Students will perform several algebraic calculations in determining the molarity of the acids.

Connect to Reading
Students will be reading about how excessive soft drink consumption can have detrimental health effects.

As an extension to this activity, possible extensions include reading about other acidic foods and their effects on health at the following links:

Connect to Writing
Students will write a reflection paper and lab report about their investigation.

For the Student

Lesson

Background

Many of the foods and beverages we consume everyday have substantial amounts of acid present in them. Acids are normally associated with giving many foods their sour or tart flavors. Common examples include vinegar, citrus juices, and soft drinks. While consuming these food items can be part of a healthy diet, excessive consumption can lead to negative health effects.

Objective

In this lab investigation you will utilize a titration to determine the citric acid content in a lemon-lime soft drink and then research possible detrimental health effects due to excessive consumption.

Materials

  • Burette, 50 mL
  • Ring stand
  • burette clamp
  • funnel
  • 0.10 M NaOH solution, 60 mL
  • Beaker, 250 mL
  • two beakers, 100 mL
  • Phenolphthalein indicator
  • Erlenmeyer flask, 125 mL
  • volumetric pipette, 25 mL
  • Pipette filler or bulb
  • Permanent marker
  • Approximate 60 mL sample of lemon-lime soft drink

Safety

  • Always wear safety goggles and lab apron when handling chemicals in the lab.
  • Wash your hands thoroughly before leaving the lab.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
  • When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
  • Do not consume lab solutions, even if they’re otherwise edible products.
  • Food in the lab should be considered a chemical not for consumption.

Procedure

Part 1: Titration of a Lemon-Lime Soft Drink

  1. First read all of the steps of this procedure to understand what data must be recorded. Create a data table in the area below titled “Data Table”. Share your data table ideas with another group and have the teacher approve/sritique it before moving forward.
  2. Label one 100 mL beaker “soda” and the other as “NaOH waste.”
  3. Place about 60 mL of clear soda into its corresponding 100 mL beaker (you will use the “NaOH waste” beaker in step 5. below).
  4. Put on nitrile gloves.
  5. Using the 250 mL beaker, obtain approximately 35-40 mL of 0.10M NaOH solution. Be very careful not to get any of this on you.
  6. You will now “prime” the burette to flush out contaminants:
  • Place the burette into the burette clamp on the ring stand.
  • Close the stopcock on the burette and carefully pour about 10 mL of the 0.10M NaOH solution from the 250 mL beaker into the burette. Use a funnel to help avoid spillage.
  • Place the 100 mL beaker labeled “NaOH waste” under the tip of the burette, and then open the stop cock to discharge all of the 0.10 M NaOH.
  1. Close the stopcock valve once again and refill the burette to between 20 and 25 mL. Once again place the 100 mL beaker labeled “NaOH waste” under the tip of the burette. Fully open the stop cock and allow a milliliter or two to drip into the waste beaker. Close the stopcock and inspect the tip of the burette to assure that no air bubbles remain. If there are any, fully open the valve until they are discharged. If the volume of the 0.10 M NaOH has dropped below 20 mL refill the burette to obtain a starting volume of at least 20 mL.
  2. Read the burette and record the starting volume of the 0.10 M NaOH.
  3. Using the 25 mL pipette and filler or bulb place 25 mL of lemon-lime soft drink into the 250 mL Erlenmeyer flask. Add 2-3 drops of phenolphthalein indicator to the flask.
  4. Place the 250 mL flask with the soft drink under the burette and slowly open the stopcock valve. You will want a fast drip of several drops per second to start. Swirl the flask as the drops are being added. The phenolphthalein in the acid solution will turn pink as each drop of 0.10 M NaOH is added. The pink color will quickly dissipate at first but after several milliliters of NaOH are added the pink color will become more and more persistent. Slow the drip rate to about a drop per second or less when it appears that the pink color is lasting more than a second or two. Once the pale pink color persists for more than a few seconds close the stopcock. Very carefully add drops one at a time. The titration is complete when you reach the endpoint. The endpoint is when a single drop of NaOH is added to the soft drink and the solution stays a pale pink color for a minute or more.
  5. Read the burette and record the final volume of the 0.10 M NaOH. You should be able to determine how the volume used during the titration.
  6. Discard the titrated soda and NaOH mixture in the flask down the sink drain, but do NOT rinse out the flask.
  7. Refill buret to 20 – 25 mL and perform another titration of soda by repeating steps 7 - 10.
  8. Clean up your lab area.

Data Table

Calculations

For this titration we assume that at the end point the number of moles of acid equal the moles of base. The titration equation below can be used to solve for the molarity of the acid (Ma) in the soda:

Ma x Va x #H’s=Mb x Vb x #OH’s

Where:
Ma = molarity of acid
Mb = molarity of base
#H’s = number of acidic hydrogens in formula
Va = volume of acid
Vb = volume of base
#OH’s = number of hydroxides in formula

  1. Use the equation above to calculate the molarity of the acid in the soft drink for each trial.Show your work below:
  1. Determine the average (mean) of the molarities for the two trials performed. Show your work below:

Procedure

Part 2: Health and Acids in Foods

Now that you have determined the molarity of the citric acid in the clear soda, it is time to begin exploring the possible health effects of soda. Although soda can be a part of a healthy lifestyle, excessive consumption of acidic foods, like sodas, can be detrimental to one’s physical well-being. Visit the following links and use the information to answer the questions that follow:

Questions

  1. What specific acids are found in soft drinks?
  2. Which acid is most abundant in colas?
  3. Which acid is most abundant in non-cola soft drinks?
  4. Many American’s enjoy soft drinks as a refreshment and the nation consumes millions of gallons every year. Though refreshing, the acids and other chemicals in them can pose potential negative health effects. List at least three specific health effects of excessive soft drink consumption below (include both effects due to acid and other chemicals in sodas):
  5. As the name implies, citric acid is found in citrus fruits and gives them their tart flavors. Citric acid has been linked to many positive health effects including fighting disease. Many parents encourage their young children to drink plenty of fruit juices due to their positive health effects. Some parents have even put citrus juice in sippy cups or baby bottles for their infant children. What are your thoughts as to the pros and cons of this practice?
  6. What are some alternatives to sodas that might be healthier choices to drink?

Conclusion:

Summarize what you have learned about titrations, the amount of acid in clear sodas, and health effects of soft drinks from this lab experience.