pH: Hands on strategies to tackle misconceptions Presenter:

Investigation 16A: What is pH? Chapter 16 What are you measuring when you measure pH?

Think About It Can you guess the pH of the following items? Justify your choices. Detergent HCl Mineral water Pure water Soda wine Soap Blood

Think About It

Safety Considerations Follow these important safety precautions in addition to your regular classroom procedures: Wear safety goggles at all times! Use caution when using acids and bases. Lab Preparation To save time in the lab, you may want to calibrate the pH sensors prior to the experiment. The following preparation instructions will produce more than enough solution for this investigation. You may be able to use leftover solution in upcoming acid-base investigations. 0.1 M Sodium hydroxide: Prepare 1000 mL of 0.1 M sodium hydroxide (NaOH) by placing 3.99 g of solid NaOH in a 1000-mL volumetric flask 1/3 full of distilled water. Stir it well and let it cool. When the solid has completely dissolved, fill the flask to the line with distilled water. 0.1 M Hydrochloric acid: To prepare 1000 mL of 0.1 M hydrochloric acid (HCl), fill a 1000-mL volumetric flask 1/3 full of distilled water. Add 8.33 mL of 12 M HCl to the flask and swirl to dissolve. Fill the flask to the line with distilled water.

What You Will Need Device with SPARKvue software / pH Sensor / Small beaker or cup Graduated cylinders (2), 10-mL / Stirring rod / Test tubes (5), 20 mm x 150 mm Test Tube rack / Pipets (2), graduated / 0.1 M HCl (10 mL) / 0.1 M NaOH (10 mL) Universal indicator solution / pH buffers (4 and 10) / Distilled water in a wash bottle Materials list missing

Set-up of Equipment

Connecting a Sensor Turn on the pH sensor and then connect it using Bluetooth.

Open the Lab File Open the 16A What is pH lab file in SPARKvue under Experiments > Essential Chemistry.

Procedure Acids Calibrate the pH sesnsor with pH 4 and 10 buffers. Label the pipets HCl and NaOH. Label test tubes with numbers 1-5. Place 10 mL of 0.10 M HCl into test tube #1. Safety Note: Recommend using a funnel when pouring the sodium hydroxide solution into the narrow buret opening.

Procedure Acids 5. Make Mixture #2: Use the HCl pipet to add 1 mL of 0.1 M HCl from test tube #1 to test tube #2. Use a clean graduated cylinder to add 9 mL of distilled water to test tube #2. Mix well. Set test tube #2 aside in the test tube rack. Calculate the concentration.

Procedure Acids 6. Make Mixture #3: Add 9 mL of distilled water to test tube #3. Use the HCl pipet to add 1 mL of mixture #2 to test tube #3. Mix the contents of the test tube well. Rinse the stirring rod with distilled water and dry it. Set test tube #3 aside in the test tube rack. Calculate the concentration.

Procedure Acids Repeat the mixture process for test tubes 4 and 5. Calculate the concentrations. Add a few drops of universal indicator and mix well. Record their colors.

Procedure Acids Use the pH sensor to measure the pH of each solution.

Analysis Acids Test tube Molarity of HCl (M) Molarity of HCl in scientific notation (M) [H+] (M) Value of the [H+] exponent Negative value of the [H+] exponent Color of solution with indicator pH 1 2 3 4 5

Analysis Acids Test tube Molarity of HCl (M) Molarity of HCl in scientific notation (M) [H+] (M) Value of the [H+] exponent Negative value of the [H+] exponent Color of solution with indicator pH 1 0.10 1 x 10-1 -1 Red 1.2 2 0.01 1 x 10-2 -2 2.1 3 0.001 1 x 10-3 -3 2.9 4 0.0001 1 x 10-4 -4 Orange 3.8 5 0.00001 1 x 10-5 -5 4.8

Questions Acids 1. What specific type of matter do you think the indicator detects? Would an indicator or a pH sensor be more useful if you needed a more accurate pH measurement of a solution? Explain your answer.

Questions Acids 1. What specific type of matter do you think the indicator detects? Indicator detects the amount of [H+] in solution. Would an indicator or a pH sensor be more useful if you needed a more accurate pH measurement of a solution? Explain your answer.

Questions Acids 1. What specific type of matter do you think the indicator detects? Indicator detects the amount of [H+] in solution. Would an indicator or a pH sensor be more useful if you needed a more accurate pH measurement of a solution? Explain your answer. It was difficult to tell the difference in colors at very low pH values. A pH sensor would be better a determining an accurate measurement because you get a number instead of a color. Colors might be interpreted or seen differently by different people.

Questions Acids Use the [H+] from Table 1 to calculate -log[H+] for each of the solutions. What column of data in Table 1 does this match? How do the calculated pH values compare to the measured pH values? what are some sources of error?

Questions Acids Use the [H+] from Table 1 to calculate -log[H+] for each of the solutions. What column of data in Table 1 does this match? The pH values match the negative value of the exponent of the concentration. The values are 1,2,3,4, and 5. How do the calculated pH values compare to the measured pH values? what are some sources of error?

Questions Acids Use the [H+] from Table 1 to calculate -log[H+] for each of the solutions. What column of data in Table 1 does this match? The pH values match the negative value of the exponent of the concentration. The values are 1,2,3,4, and 5. How do the calculated pH values compare to the measured pH values? what are some sources of error? Answers will vary. Mistakes made while collecting volumes of water or solutions during dilution and pH sensor calibration could be indicated as possible sources of error.

Questions Acids If your solution had a [H+] equal to 2 x 10-5 M, what is the approximate pH based on the exponent? Using your calculator, what is the actual pH?

Questions Acids If your solution had a [H+] equal to 2 x 10-5 M, what is the approximate pH based on the exponent? Using your calculator, what is the actual pH? Based on the exponent it should be about a pH of 5. Using the calculator the pH is 4.70.

Procedure Bases Clean and rinse the test tubes and graduated cylinders. Label the 5 test tubes with numbers #9 to #13 and place them in the test tube rack. Pour about 10 mL of 0.1 M NaOH into the test tube labeled #13. Set this aside in the test tube rack.

Procedure Bases 3. Make Mixture #12: Use a clean graduated cylinder to add 9 mL of distilled water to test tube #12. Use a pipet to add 1 mL of 0.1 M NaOH from test tube #13 to test tube #12. Mix the solution well. Calculate the concentration

Procedure Bases 4. Make Mixture #11: Add 9 mL of distilled water to test tube #11. Use the NaOH pipet to add 1 mL of mixture #12 to test tube #11. Mix the solution well. Calculate the concentration

Procedure Bases Make mixtures #10 and #9 by repeating the previous dilution. To make each solution, add 1 mL of the previous numbered solution to 9 mL of distilled water making a total volume of 10 mL. Calculate the concentrations. Add a few drops of universal indicator.

Procedure Bases Use the pH sensor to measure the pH of each solution.

Analysis Bases Test tube Molarity of NaOH (M) Molarity of NaOH in scientific notation (M) [OH-] (M) Value of the [OH-] exponent Negative value of the [OH-] exponent Color of solution with indicator pH 13 12 11 10 9

Analysis Bases Test tube Molarity of NaOH (M) Molarity of NaOH in scientific notation (M) [OH-] (M) Value of the [OH-] exponent Negative value of the [OH-] exponent Color of solution with indicator pH 13 0.10 1 x 10-1 -1 1 Purple 12.5 12 0.01 1 x 10-2 -2 2 11.9 11 0.001 1 x 10-3 -3 3 Blue 10.8 10 0.0001 1 x 10-4 -4 4 9.7 9 0.00001 1 x 10-5 -5 5 Blue-green 8.8

Questions Bases Now that you have experience working with both acids and bases, what do you think the indicator detects? Explain your answer. pOH is defined as the –log[OH-]. Calculate the pOH for each of the test tubes. What column of data Table 2 does this represent.

Questions Bases Now that you have experience working with both acids and bases, what do you think the indicator detects? Explain your answer. The indicator changed color as before but the colors are different for bases. The indicator might be able to detect H+ as well as OH-. pOH is defined as the –log[OH-]. Calculate the pOH for each of the test tubes. What column of data Table 2 does this represent.

Questions Bases Now that you have experience working with both acids and bases, what do you think the indicator detects? Explain your answer. The indicator changed color as before but the colors are different for bases. The indicator might be able to detect H+ as well as OH-. pOH is defined as the –log[OH-]. Calculate the pOH for each of the test tubes. What column of data Table 2 does this represent. The pOH is 1, 2, 3, 4, and 5 respectively. This number matches the negative of the exponent of the concentration of OH-.

Questions Bases 3. pOH is related to pH through the following equation: 14 = pH + pOH What are the expected pH values for each of your test tubes?

Questions Bases 3. pOH is related to pH through the following equation: 14 = pH + pOH What are the expected pH values for each of your test tubes? The expected pH’s are 13, 12, 11, 10, and 9.

Questions Bases How do the expected pH values based on exponents compare to the pH values measured with the sensor? If your solution had a concentration of 2 x 10-5 M OH-, what is the value of the pOH and the value of the pH?

Questions Bases How do the expected pH values based on exponents compare to the pH values measured with the sensor? The measured values are close to the expected values. If your solution had a concentration of 2 x 10-5 M OH-, what is the value of the pOH and the value of the pH?

Questions Bases How do the expected pH values based on exponents compare to the pH values measured with the sensor? The measured values are close to the expected values. If your solution had a concentration of 2 x 10-5 M OH-, what is the value of the pOH and the value of the pH? pOH is 4.70 and the pH would be 14 - 4.7 = 9.3.

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