15. Acid-Base Titration. Discover the concentration of an unknown acid solution using acid-base titration.

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S t u d e n t I n s t r u c t i o n S h e e t 375

Shift into Neutral

Student Instruction Sheet

Challenge

Discover the concentration of an unknown acid solution using acid-base titration.

Equipment and Materials

Safety Precautions

Remember, follow the directions for using the equipment.

Wear safety glasses and follow standard laboratory safety procedures.

Keep liquids away from electrical outlets, the computer, the keyboard, and the PASPORT equipment.

Do not pipette by mouth.

• computer with USB port • hydrochloric acid (HCl), unknown

• PASPORT USB interface concentration, 10.0 mL

• PASPORT pH Sensor • magnetic stirrer and stir bar

• DataStudio software • pipet with bulb, 10-mL

• Small Tripod Base & Rod • protective gear

• Buret Clamps, 2 • wash bottle and waste container

• beaker, 250-mL • water, distilled, 500.0 mL

• buret, 50-mL • Student Instruction Sheet

• 0.10 M sodium hydroxide (NaOH), 100.0 mL • Student Response Sheet

• graduated cylinder, 100-mL

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376 S t u d e n t I n s t r u c t io n S h e e t

Background

The pH of a solution indicates whether it is acidic, basic (alkaline), or neutral.

Titration is a quantitative measurement of an unknown solution (called the analyte) by reacting it completely with a standardized reagent that is a solution of known concentration (called the titrant). Acids and bases react until one of the reactants is consumed completely. A base solution of known concentration can therefore be used to titrate an acid solution of unknown concentration. As a base solution is added to an acid solution, ions from the base and ions from the acid solution react chemically to form a neutral product. As a result, the pH of the acid solution begins to change. The equivalence point is the point at which all the ions from the acid solution are bonded to ions from the base solution. The

equivalence point of an acid-base titration can be detected with a pH electrode.

Predict

Before beginning the eLab, complete the prediction portion and define the vocabulary words on the Student Response Sheet.

Explore

Computer Setup

1. Plug the USB interface into the computer’s USB port.

2. Connect the pH electrode to the pH Sensor. Plug the pH Sensor into the USB interface.

This will automatically

launch the PASPORTAL window.

3. Choose the appropriate DataStudio configuration file entitled 15 Titration CF.ds.

and proceed with the following instructions.

Note:

Configuration files automatically launch the appropriate

display(s), sampling rate(s). etc.

To computer

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S t u d e n t I n s t r u c t i o n S h e e t 377 Equipment Setup

1. Set up the equipment as shown in the diagram. For this part you will need the following: hydrochloric acid solution, distilled water, 250-mL beaker, 50-mL buret, pH Sensor, buret clamps, small tripod base & rod, magnetic stirrer and stir bar, 0.10 M sodium hydroxide (NaOH) solution.

• Measure 100.0 mL of distilled water into a clean, dry 250-mL beaker.

• Use a pipet to add 10.0 mL of the hydrochloric acid solution into the beaker with the distilled water.

Record the volume of hydrochloric acid in Table 1 on the Student Response Sheet.

CAUTION:

Never pipet by mouth. Always use a

pipet bulb or a pipet pump. Be careful when handling

any acid or base solutions.

• Carefully add a spin bar to the beaker. Place the beaker on the magnetic stirrer.

• Use a clamp and small tripod base & rod to position the pH electrode so that the end of the electrode is in contact with the acid solution, but is not

interfering with the spin bar.

2. Rinse the 50-mL buret with a few milliliters of the 0.10 M sodium hydroxide solution. Dispose of the rinse solution as directed.

3. Use a second clamp to support the 50-mL buret so the end of the buret is above the acid solution. (Be sure the buret valve is closed!)

4. Fill the buret with 0.10 M sodium hydroxide solution. Be sure to start the titration with the buret filled exactly to the 0.00 mL mark. Record the precise concentration of the sodium hydroxide solution in Table 1 on the Student Response Sheet.

5. Turn on the magnetic stirrer. (If you are not using a magnetic stirrer, use a stirring rod.)

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378 S t u d e n t I n s t r u c t io n S h e e t Record Data

1. Arrange the displays so you can see the table of “pH and Volume”.

2. Click the Start ( ) button to begin recording data. The Start button changes to a Keep ( ) button.

• The first row of the table shows the beginning value of pH.

Note:

Do not add any NaOH titrant for the first reading.

3. When the pH value stabilizes, click Keep. Click the Tab key to move to the next row in the Table.

4. The Table display shows the first 'pH, Volume' pair in the first row.

5. Open the buret's valve and add some of the sodium hydroxide. Add enough titrant to raise the pH by about 0.15 units, and then close the valve.

6. After the titrant has been added and the pH reading stabilizes, type the total volume of sodium hydroxide added into the next row in the table and then click Keep.

Note

: To read the volume, measure at the bottom of the

meniscus of the sodium hydroxide solution in the buret.

7. Continue adding sodium hydroxide solution in amounts that raise the pH by about 0.15 units. Each time type in the total amount of sodium hydroxide added.

8. When pH 3.5 is reached, change to 2-drop increments. Continue to type in the total volume of sodium hydroxide added after each increment.

9. After pH 10.5 is reached, again add larger increments of sodium hydroxide (enough to raise the pH by about 0.15 units) and type in the total volume of sodium hydroxide added.

10. Continue adding sodium hydroxide until the pH stops changing. Click the Stop ( ) button to end data recording.

11. Turn off the magnetic stirrer. Remove the pH Sensor from the solution. Rinse the pH Sensor in distilled water and dry the sensor gently.

12. Dispose of the solution in the beaker as instructed.

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S t u d e n t I n s t r u c t i o n S h e e t 379 13. If time permits, repeat the procedure.

14. Save your DataStudio file (on the File menu, click Save Activity As...) to the location specified by your teacher.

Analyze

Record calculations in your data table on the Student Response Sheet as you complete your analysis.

1. In the graph display, click the Smart Tool ( ) button. Use the Smart Tool ( ) cursor to find the value of the pH in the plot of pH versus Volume just before the largest pH increase. Record the x coordinate as the "NaOH volume added before largest pH increase" in Table 1 on the Student Response Sheet.

2. Move the Smart Tool cursor to the place in the plot just after the largest pH increase. Record the x coordinate as the "NaOH volume added after largest pH increase" in Table 1 on the Student Response Sheet.

3. Move the Smart Tool cursor to the point on the plot where the pH is 7.0 (or as close as possible). Record the x-coordinate as the "NaOH volume added at the equivalence point” in Table 2 on the Student Response Sheet.

4. The average of the volume of NaOH just before and just after the largest pH increase should be very close to the volume of NaOH added at pH = 7.

5. Answer all the questions on the Student Response Sheet.

6. Follow your teacher’s instructions regarding cleaning up your work space.

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380 S t u d e n t I n s t r u c t io n S h e e t PS-2808

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S t u d e n t R e s p o n s e S h e e t 385

Student Response Sheet

Name:___________________________________

Date: ___________________________________

Shift into Neutral

Vocabulary

Use available resources to find the definitions of the following terms:

analyte: _____________________________________________________

___________________________________________________________

buret: _______________________________________________________

___________________________________________________________

equivalence point: ______________________________________________

___________________________________________________________

molarity: _____________________________________________________

___________________________________________________________

titrant: ______________________________________________________

___________________________________________________________

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386 S t u d e n t R e s p o n s e S h e e t

Predict

1. What will be the pH value of the solution formed when enough strong base solution is added to a strong acid solution so that the solution reaches the equivalence point?

___________________________________________________________

2. The acid you will use in this experiment is hydrochloric acid and the base is sodium hydroxide. Write the balanced chemical equation for this reaction.

___________________________________________________________

3. What is the ratio of acid to base in this reaction?

___________________________________________________________

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Data

Make a sketch of your graph of pH versus Volume.

In your graph of pH versus Volume, use the Smart Tool to find the volume of titrant (sodium hydroxide solution) just before, just after, and at the equivalence point. Record your results in Table 1.

Table 1

Item Value

Volume of HCl

Concentration of NaOH

NaOH volume added before largest pH increase

NaOH volume added after largest pH increase

Average of NaOH before and after pH increase

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388 S t u d e n t R e s p o n s e S h e e t

Analyze

1. Use the values from Table 1, calculate the number of moles of

NaOH

^used.

Record the number in Table 2 below.

___________________________________________________________

2. Use the equation for the neutralization reaction to calculate the number of moles of HCl used. Record the number in Table 2 below.

• Recall that you pipetted out 10.0 mL of the unknown HCl solution for the titration.

___________________________________________________________

3. Calculate the HCl concentration in mole/liter (mol/L). Record the concentration in Table 2 below.

Table 2

Item Value

NaOH volume added at equivalence point

Moles of NaOH

Moles of HCl

Volume of HCl

Concentration of HCl

Number of moles of NaOH = volume¥molarity

Number of moles of HCl = number of moles of NaOH

molarity HCl number of moles HCl volume of HCl ---

=

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Synthesize

1. What is the concentration of HCl you discovered?

___________________________________________________________

2. From your teacher, find the “unknown” HCl solution? Determine how close to this value your calculated molarity is.

___________________________________________________________

3. Do your results support your predictions?

___________________________________________________________

4. Consider the table below:

If substance B is NaOH, which of the following could be substance A? Why?

a. HClO₄ b. H₂SO₄ c. H₃PO₄ d. HNO₃

____________________________________________

___________________________________________________________

Item Value

Moles of substance A 0.020 mol

moles of substance B 0.040 mol

concentration of B 4.0 M

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390 S t u d e n t R e s p o n s e S h e e t PS-2808