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LAB in Chemical Change, Concentration, Acid Base Reactions, Molarity, Balancing Equations, Stoichiometry, Dimensional Analysis, Measurements, Titrations, Indicators, Error Analysis, Chemical Technical Professionals. Last updated October 24, 2019.

Summary

In this lab, students will perform a titration of a vinegar sample to determine if it is it close to the concentration claimed on the bottle.

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

  • perform an acid/base titration using burettes.
  • write a simple acid/base reaction.
  • calculate the concentration of a solution (acetic acid in vinegar).
  • evaluate whether an experimental value is within allowable error from an accepted value.

Chemistry Topics

  • Acids and Bases
  • Titrations
  • Balancing Chemical Equations
  • Stoichiometry

Time

Teacher Preparation: 1 hour

Lesson: 1 hour

Materials

  • store bought bottle of vinegar - roughly 50 mL per lab group
  • 0.50 M solution of sodium hydroxide (20.0 g of NaOH per each 1 L of solution) - roughly 60 mL per lab group. This can be prepared and standardized using KHP a day or two in advance.
  • 2 burettes along with ring stand and burette clamp
  • 125 mL Erlenmeyer flask
  • 250 mL beakers (2)
  • student safety goggles and lab aprons

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
  • 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.

Teacher Notes

  • Be sure to demonstrate the proper technique for using burettes (either the day prior or, if you have advanced/efficient students, the day of the lab)
  • In my accelerated first-year chemistry classroom this activity comes during our second semester after we have discussed simple acid/base reactions for a couple of days. By this point stoichiometry has been taught and used numerous times throughout the year so that the vast majority of my students are quite comfortable with this calculation method for reactions. For my non-accelerated chemistry classes we typically work through a sample set of data as part of helping them to perform their lab calculations.
  • Disposal: It is safe to dispose of all solutions down the drain.
  • A suggestion for added entertainment value: secretly dilute the vinegar solution so that it does not end up giving the expected acidity (particularly effective if you have more than one section of chemistry during the day, so the student’s results are not the same!)

For the Student

Lesson

Background

Typically the vinegar sold in stores is very close to 5% acetic acid in water. One way of determining this concentration is by performing a titration. This is a way of using solution stoichiometry for an acid/base reaction. In this lab, you will test a vinegar sample by performing a titration with a sodium hydroxide solution of known concentration to see if a sample of vinegar will "pass inspection."

Materials

  • 2 burettes
  • ring stand & burette clamp
  • 125 Erlenmeyer flask
  • 250 mL beakers (2)
  • 60ml of 0.50M Sodium Hydroxide (NaOH)
  • 50ml of Vinegar
  • Phenolphthalein indicator

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
  • Wash your hands thoroughly before leaving the lab.
  • All solutions can be poured down the drain when the lab in completed.

Procedure

  1. Set up the ring stand and burette clap in your lab station. The 125ml Erlenmeyer flask will be used to collect the sample during the titration.
  2. Use a clean beaker and obtain about 50 mL of vinegar solution. Rinse out one of your burettes with some water and then rinse with some of the vinegar solution. Fill your burette with the vinegar solution so that it goes just past the 0 mL mark. Drain the burette down until the vinegar is level at the 0 mL mark (it can be drained into the beaker). Record this 0 mL into the data table as your starting volume. Read the meniscus of the vinegar, estimate if the meniscus is not exactly on the line, and record your starting volume to two decimal places.
  3. Repeat the above step for the sodium hydroxide (NaOH) solution. Be sure to also record the molarity of the NaOH solution into the data table.
  4. Drain about 10 mL of vinegar solution from the burette into the empty Erlenmeyer flask. Add two drops of phenolphthalein indicator to the vinegar in the flask (the vinegar should remain clear and colorless once the phenolphthalein is added).
  5. Titrate the sample by slowly adding base from the burette to the acid in the flask. Do this until it takes only a drop or two for the indicator to change to a permanent pink color.
  6. Read the volumes from both burettes and record these final volumes into the data table (proper measurement should show you that this can be recorded to two placed past the decimal).
  7. Repeat steps 1 - 5 at least two more times so as to obtain average values for your volumes used. You should have three consistent trials before considering your experiment finished. If time permits do as many trials as possible; the more trials the more precise your average should be.

Vinegar Burette Initial value

Vinegar Burette Final value

Base Burette Initial value

Base Burette Final value

Molarity of Base (from bottle)

Trial I

Trial II

Trial III

Trial IV (if needed)

Data

Qualitative observations (what did you notice during the experiment?)

Calculations

Be sure to write any work for the calculations as part of supporting your answers!

  1. For each trial calculate the volume of vinegar used. Then repeat this for the volume of base used. Record your results in the table below and then find the averages.

Vinegar Volume

Base Volume

Trial I

Trial II

Trial III

Trial IV

Average

  1. Write out the balanced equation of the reaction of the vinegar (acetic acid) with the base (sodium hydroxide) to form sodium acetate and water. Be sure to include physical states in the final equation.
  2. Use the average base volume as your starting point and perform the stoichiometry that will give the experimental molarity of the vinegar.

Analysis

  1. Normal store-bought vinegar has a concentration of around 0.85 M acetic acid (that's very close to the 5% acidity as stated on the label). Find the percent error of your calculated lab value from this acceptable value of 0.85 M. Discuss whether or not your feel that your sample would pass inspection (be sure to explain your reasoning!).