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LAB in pH, Titrations, Indicators, Strong vs Weak. Last updated April 26, 2019.


Summary

In this lab, students will learn the difference between strong, weak, and concentrated acids by carrying out titrations.

Grade Level

High school

AP Chemistry Curriculum Framework

This lab supports the following learning objectives:

  • Big Idea 3: Changes in matter involve the rearrangement and/or reorganization of atoms and/or the transfer of electrons.
    • 3.10 The student is able to evaluate the classification of a process as a physical change, chemical change, or ambiguous change based on both macroscopic observations and the distinction between rearrangement of covalent interactions and non-covalent interactions.
    • 3.2 The student can translate an observed chemical change into a balanced chemical equation and justify the choice of equation type (molecular, ionic, or net ionic) in terms of utility for the given circumstances.
    • 3.3 The student is able to use stoichiometric calculations to predict the results of performing a reaction in the laboratory and/or to analyze deviations from the expected results.
    • 3.4 The student is able to relate quantities (measured mass of substances, volumes of solutions, or volumes and pressures of gases) to identify stoichiometric relationships for a reaction, including situations involving limiting reactants and situations in which the reaction has not gone to completion.
  • Big Idea 6: Any bond or intermolecular attraction that can be formed can be broken. These two processes are in a dynamic competition, sensitive to initial conditions and external perturbations.
    • 6.13 The student can interpret titration data for monoprotic or polyprotic acids involving titration of a weak or strong acid by a strong base (or a weak or strong base by a strong acid) to determine the concentration of the titrant and the pKa for a weak acid, or the pKb for a weak base.

Objectives

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

  • Differentiate between the phrases strong acid and concentrated acid.
  • Carry out a titration.

Chemistry Topics

This lesson supports students’ understanding of

  • Strong vs. weak acids/bases
  • Indicators
  • pH
  • titrations

Time

Teacher Preparation: 1 hour

Lesson: 1 class period

Materials

For each group:

  • Buret (titration apparatus)
  • HCl (pH = 2.6)
  • CH3COOH (pH = 2.6)
  • KOH (pH = 11.6)
  • Two Erlenmeyer flasks (250 mL)
  • Indicator and/or pH meter

Safety

  • Always wear safety goggles when working in the lab.
  • When working with acids and bases, if any solution gets on your skin be sure to thoroughly flush it with water immediately.
  • 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 chemicalsly.

Teacher Notes

  • Review strong vs. weak acids and bases PowerPoint with students before doing the lab investigation.
  • Information about acids and bases that students need to know for this activity:

    pH pH measures the hydrogen ion concentration of a solution. You may have thought that it measures the strength of an acid, but—to a chemist—the strength of an acid is something else. The most dangerous acid solutions have a pH that is close to zero. The most dangerous bases have a pH that is close to 14.

    pOH
    You might say that pOH is the opposite of pH. pOH measures the hydroxide ion concentration of a solution. The most dangerous base solutions have a pOH that are close to zero. The most dangerous acids have a pOH that is close to 14.

    Indicator – These are chemicals that change color as the pH or pOH is changed.

    pH meter – This a device that measures the pH of a solution.

For the Student

Lesson

Background

Write and number everything that you can remember about acids and bases in three minutes.

Purpose

You will determine whether it takes more base to neutralize two different acid solutions that have the same pH (one is a strong acid, one is a weak acid).

Prelab Questions

Which of the following 3 choices do you believe is true? Check one.

  • It will require more sodium hydroxide solution to neutralize pH 2.6 hydrochloric acid.
  • It will require more sodium hydroxide solution to neutralize pH 2.6 acetic acid.
  • It will require the same amount of sodium hydroxide solution to neutralize both acids.

In the space provided below explain why you made the prediction above.

In the space provided below explain how you would test your prediction above.

Procedure

  1. Measure 10.0 mL of acetic acid and 10.0 mL of hydrochloric acid into container of your choice.
  2. You will be given a large volume of base solution.
  3. You and your lab group will need to develop a set of procedures to test whether it takes more base to neutralize a strong acid or a weak acid. Your group can begin your lab once every member of your lab group has the procedures outlined.
  4. Both acids and bases are neutralized when the pH reaches 7. You may be using a universal pH indicator or a pH meter to know when your acid solution has been neutralized. Your teacher will show you how to use a pH meter, if that is what you are using. Use the chart on the bottle of universal indicator to see how the color changes to show the approximate pH.
  5. Write complete observations from your lab. You are responsible for your own lab observations.

Observations




Analysis

How were your observations different from your predictions and explain why you believe that these differences might have occurred.

Conclusion

Write down what new things you learned from this activity.