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DEMONSTRATION in Concentration, Molarity, Titrations, Indicators, Unlocked Resources. Last updated October 03, 2024.

Summary

In this demonstration, the teacher will show how a titration is set-up and performed. The teacher will utilize different indicators to show how they work and why they are necessary. At the end of the demonstration, the teacher will also explain how to calculate the molarity of the unknown substance.

Grade Level

High School

AP Chemistry Curriculum Framework

This demonstration supports the following units, topics, and learning objectives:

  • Unit 4: Chemical Reactions
    • Topic 4.2: Net Ionic Equations
      • 4.2.A: 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
      • 4.5.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
      • 4.6.A: 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 8.5: Acid-Base Titrations
      • 8.5.A: 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.

Objectives

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

  • Understand how to set-up the equipment for a titration.
  • Accurately follow the procedural steps to perform a titration.
  • Identify different indicators and explain their purpose.
  • Calculate the molarity of an unknown substance.

Chemistry Topics

This lesson supports students’ understanding of

  • Acids and bases
  • Titrations
  • Indicators
  • pH
  • Molarity

Time

Teacher Preparation: 30- 45 minutes

Lesson: 45 minutes

Materials

  • 4 burets
  • 2 ring stands
  • 2 buret clamps
  • 4 flasks
  • 0.1- 0.25M Hydrochloric acid (~40mL per class)
  • 0.25M Sodium Hydroxide (~50mL per class)
  • Distilled water (~80mL per class)
  • Phenolphthalein
  • Bromothymol Blue
  • Universal Indicator
  • 3 pipets (for the indicator)
  • pH paper
  • Graduated cylinders

Safety

  • Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required
  • When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
  • When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
  • Always wear safety goggles 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 diluting acids, always add acid to water.

Teacher Notes

  • Prior to doing this demonstration, students should be taught about titrations and indicators, so that they have the necessary background information, and can understand the purpose of the demonstration.
  • I demonstrate and explain the titrations with the indicators added to them first and then discuss the example that does not have an indicator added.
  • Most of the time I don’t need to demonstrate the titration with no indicator added, because they already understand and can predict, based on our discussion, that they won’t be able to see anything happen.
  • The demonstration procedures are included on the student document, so that the student can have the titration procedures for future reference.
  • Students should follow along with the procedures, as given to them on the student document and record the necessary data as each step is demonstrated for them.
  • It might help to label each flask with a number for each titration.
  • For each of these titration it should not take more than 7mL of the base to titrate the acid.
  • Possible answer to conclusion question: The shampoo must have some type of indicator in it, most likely Bromothymol blue. Bromothymol blue is blue when in the presence of a base, and sodium hydroxide is a base. So, as long as there is still relaxer in the woman’s hair the soap from the shampoo will be blue. Once the soap is colorless, that lets the stylist know that all of the relaxer has been washed out.

For the Student

Background

The process of titration will be used to determine the concentration of a solution. At the equivalence point there will be an equal number of moles of H3O+ and OH- ions. We will use various indicators to detect the equivalence point through the observation of a color change. This can easily be determined because the solution will change color at the equivalence point by the addition of a single drop of base. At the equivalence point, or neutralization, the number of moles of acid equals the number of moles of base, if the mole ratio of the balanced equation is one.

Objective

In this demonstration, you will observe how a titration is set-up and performed. Also, you will be able to calculate the molarity of the unknown substance.

Materials

  • Burets
  • Ring Stands
  • Buret Clamps
  • Flasks
  • Hydrochloric acid
  • 0.25M Sodium Hydroxide
  • Distilled water
  • Phenolphthalein
  • Bromothymol Blue
  • Universal Indicator
  • Pipets
  • pH paper

Safety

  • Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required
  • When working with acids, if any solution gets on your skin, immediately alert your teacher and thoroughly flush their skin with water.
  • Wash your hands thoroughly before leaving the lab.
  • When diluting acids, always add acid to water.

Procedure

  1. The burette contains 0.25M NaOH solution.
  2. Record the initial reading of the burette in the data table . This is the NaOH initial.
  3. Measure 20 ml of distilled water to increase the volume for each of the 4 flasks and then measure and add 10 ml of HCl solution to each.
  4. Add 2-3 drops of the phenolphthalein indicator to the flask 1. Use a swirling motion to mix the solutions.
  5. Add 2-3 Drops of Universal Indicator into flask 2. Use a swirling motion to mix the solutions.
  6. Add 2-3 Drops of Brothymol Blue into flask 3. Use a swirling motion to mix the solutions.
  7. No indicator is added to flask 4.
  8. HCl initial is recorded as 0ml and HCl final is recorded as 10ml resulting in a 10ml volume of HCl used.
  9. Obtain a small piece of pH paper and determine the pH of the solution in the flask. Record the pH based on the color change of the paper by checking its color with scale on the container. Record the pH value in the data table as “pH initial.”
  10. Place the flask under the NaOH burette and add the base slowly, drop by drop while swirling the solution.
  11. Watch the solution carefully, when the last drop added causes the solution to change color the titration is close to completion. If the solution changes color, and remains this color after swirling, stop the titration.
  12. Using pH paper, again determine the pH of the solution and record the value in the data table as “pH final.”
  13. Record the final reading from the burette in the data table as “NaOH final.”
  14. Repeat these procedures for each of the examples.

*Note: DO NOT ADD MORE THAN 7 ML of NaOH to ANY of the flasks.

Data

Trial HCl Initial HCl Final Volume HCl NaOH Initial NaOH Final Volume NaOH
(final-initial) 		pH Initial pH Final
Phenolphthalein
Indicator 		0mL 10mL 10mL
Universal Indicator 0mL 10mL 10mL
Brothymol Blue
Indicator 		0mL 10mL 10mL
No Indicator Added (Control) 0mL 10mL 10mL

Calculations

Use the information from the data table and the following equation to calculate the Molarity of the acid.

[Molarity (acid) × Volume (acid) × #of Hydrogens = Molarity (base) × Volume (base) × # of Hydroxides]

Analysis

  1. Are the molarity values that you calculated in a reasonable range? Why or why not?
  2. Which indicator do you think worked the best? Explain why.

Conclusion

A woman went to a salon to get a relaxer put on her hair. Relaxers are made with sodium hydroxide. After washing the relaxer out the stylist needs to shampoo the woman’s hair again. The soap from the shampoo is blue. The stylist shampoos the woman’s hair again, but this time the soap is a little less blue. The stylist shampoos the woman’s hair a third time and this time the soap is colorless. What caused the shampoo to change colors?