ZERO-IN

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The non-italicized font represents additional information included to support the teacher’s understanding of the content being introduced within the CELL.

MINDSET

This Investigation is designed to:

• allow students to investigate the neutralization reaction between an acid and a base,
• allow students to correlate the pH of a solution with the concentration of hydrogen ions,
• allow students to correlate the amount of one reactant with the decrease in the amount of the other reactant, and
• allow students to investigate what occurs in a chemical reaction when one of two reactants is present in a limiting amount.

BE PREPARED

Teacher Preparation for the Investigation includes the following. This preparation should be done prior to students arriving in the lab.

• Prepare the 0.1N hydrochloric acid (HCl) according to the following table.

Note: If spills occur, rinse the skin, piece of clothing or table with water. Care should be exercised to prevent splashing any solution into the eyes.

• For each student group, pour 200 ml of the 0.1N hydrochloric acid (HCl) into a 400 ml beaker.
• For each student group, pour 200 ml of 0.1N (0.1M) sodium hydroxide (NaOH) into a 400 ml beaker.
• Place all materials at a central location.
• Divide students into five cooperative groups.

Note: Each student lab group will need the materials listed below.

Student Preparation for the Investigation includes having students gather the following materials. This preparation takes place on lab day after student lab groups have settled at their assigned lab tables.

Note: The materials are listed in students’ SDRs. They are also listed below for your reference.

• (1) pair of goggles per student
• (1) pair of gloves per student
• (1) hotplate/stirrer
• (1) stir bar
• (1) mortar and pestle
• (1) triple beam balance
• (1) weigh dish
• (2) antacid tablets
• (1) lab scoop
• (1) test tube rack
• (1) 15 ml centrifuge tube
• (1) dropper pipette
• (1) box of pH paper strips
• (1) lab marker
• (1) 400 ml beaker containing 200 ml of 0.1N HCl
• (1) 400 ml beaker containing 200 ml of 0.1N (0.1M) NaOH
• (1) 100 ml graduated cylinder
• (1) piece of plastic wrap 30cm long

Direct one student from each lab group to collect the materials listed in their SDRs.

INVESTIGATE

• Inform students that they will perform two Trials in this Investigation. 
  • In Trial 1, students will neutralize a sample of hydrochloric acid the same concentration as stomach acid using antacid tablets containing magnesium hydroxide. Students will determine the pH of the solution before and after the addition of the antacid tablets. Students should record their data in Table A.
  • In Trial 2, students will neutralize a sample of hydrochloric acid by repeatedly adding small volumes of sodium hydroxide solution. Students will determine the pH after each addition of sodium hydroxide. Students should record their data in Table B.
• Encourage students to reflect on the PreLab video as they move through the procedural steps.
• Explain to students that during the Experiment, every procedural step is important. If one step is skipped, data can become invalid. To help students keep on track, direct them to read each step thoroughly, complete the step, then check it off (Read it – Do it – Check it off).
• Direct students to complete the procedural steps in their SDRs.

Note: The procedural steps are listed below for your reference. Teacher “Notes” are inserted, as needed, to help facilitate the lab.

Trial 1:

1. In this Trial, you will neutralize a solution of hydrochloric acid using an antacid medication which is considered a base.
2. Use the 100 ml graduated cylinder to measure 100ml of 0.1N HCl and pour it into the 400 ml beaker.
3. Using the mortar and pestle, grind two antacid tablets into a fine powder
4. Use the triple beam balance, a lab scoop and a weigh dish to measure 2g of the powdered antacid.
5. Spread the plastic wrap on the table next to the hotplate/stirrer.
6. Dip ¼ of the pH paper strip into the HCl solution. Place the strip on the plastic wrap. Record the pH in Table A.
7. Make a prediction. Will adding the antacid medication change the pH of the HCl solution? Student answers will vary. The pH will rise into the neutral range.
8. Add a stir bar to the beaker and place it on the hotplate/stirrer. Turn the stirrer control to 3 or 300 rpm. Try to avoid splashing HCl onto the sides of the beaker.

9. Using the lab scoop, add 2 g of the powdered antacid to the HCl solution. Let the mixture stir until all of the powder is dissolved. Turn off the hotplate/stirrer.
10. Use a second pH paper strip and dip ¼ of the strip into the mixture. Place the strip to the right of the first strip.
11. Determine the pH of the HCl before and after adding the antacid by comparing the pH strips to the pictures on the pH chart. In Table A, record the pH of the HCl before and after you added the powdered antacid.

12. When you are done, rinse the 400ml beaker with tap water.
13. What was the result of adding the antacid powder to the HCl? The pH increased from 1 to 5.
14. What happened to the H⁺ ion concentration when the antacid powder was added? The H⁺ ion concentration decreased.

Trial 2:

1. In this Trial, you will neutralize a solution of hydrochloric acid (HCl) using a solution of sodium hydroxide (NaOH).
2. Use the 100ml graduated cylinder to measure 100ml of 0.1N HCl and pour it into the 400 ml beaker.
3. Add a stir bar to the beaker and place it on the hotplate/stirrer. Turn the speed control to 3. Be careful to avoid splashing the HCl onto the sides of the beaker.
4. Spread the plastic wrap on the table next to the hotplate/stirrer.
5. Label the tops of 16 pH paper strips with the numbers from 1 to 16.
6. You will determine the pH of the solution each time you add 10ml of the sodium hydroxide (NaOH) solution to the hydrochloric acid (HCl) solution. Refer to Table A to help you with the following procedure.

A. Dip ¼ of pH paper strip 1 into the hydrochloric acid (HCl) solution. Place pH paper strip 1 just inside the lower-left corner of the plastic wrap.

B. Using the dropper pipette, add 10ml of sodium hydroxide (NaOH) solution to the centrifuge tube.

C. Pour the sodium hydroxide (NaOH) into the beaker on the hotplate/stirrer.

D. Dip ¼ of pH paper strip 2 into the HCl solution. Remove the pH paper strip and place it to the right of strip 1.

E. Using the dropper pipette, add 10ml of sodium hydroxide (NaOH) solution to the centrifuge tube.

F. Pour the sodium hydroxide (NaOH) into the beaker on the hotplate/stirrer.

G. Dip ¼ of pH paper strip 3 into the HCl solution. Remove the pH paper strip and place it to the right of strip 2.

H. Repeat this procedure until you have lined up all 16 pH paper strips on the plastic wrap as shown in the picture. Fold the excess plastic wrap down over the strips.

7. Match the color on each pH paper strip with the pictures of the squares on the pH chart. Estimate the pH to the nearest half of a pH unit, for example, 3.0, 3.5, or 4.0.
8. Record the pH values for all 16 pH strips in Table A.

CLEAN UP

Let students know your expectations for clean up. Ask them to clean up.