Chemistry e-lab:Acid-Base Titration N. Jahng
Overview of the Activity • Students will use the pH Sensor to investigate the titration process. They will use a titrant of a known concentration to determine the concentration of a HCI solution.
Learning Goals Students will: • Learn the relationship between pH and chemical reactions. • Learn how the titration process works. • Understand how to use titration to determine molarity. • Observe the curve of a titration-inspect how pH change is gradual on either side of the equivalence point until very near it. • Use the plot of pH versus volume to determine the amount of sodium hydroxide solution needed to neutralize the acid solution.
Vocabulary • pH indicates whether a solution is acidic, basic (alkaline), or neutral. • Titration is a quantitative measurement of an unknown solution (called the analyte: the substance being analyzed) by reacting it completely with a standardized reagent that is a solution of known concentration (called the titrant). • Endpoint: the point when the indicator colour changes (usually the end of the titration) • Eqivalence point in a titrantion process at which the amount of titrant is chemically equivalent to the amount of material titrated (in acid-base titrations equivalents refer to the number of moles of H+ and OH–) • Indicator: A compound having a physical property (usually color) that changes abruptly near the equivalence point of a chemical reaction. • Molarity is a description of the concentration of a solution, expressed by the number of moles of solute per liter of solution.
Background • 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 (sodium hydroxide, NaOH) is added to an acid solution (hydrochloric acid, HCI), 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. • YouTube video
Computer Setup • Plug the USB interface into the computer’s USB port. • Connect the pH electrode to the pH Sensor. Plug the pH Sensor into the USB interface. This will automatically launch the PASPORTAL window. pH Sensor USB interface pH electrode
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.
Equipment Setup Rinse and fill the buret with 0.10 M sodium hydroxide solution 10.0 mL of the hydrochloric acid solution 100.0 mL of distilled water pH electrode magnetic stirrer Titration Technique using a buret
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. • Carefully add a spin bar to the beaker. Place the beaker on the magnetic stirrer. • Position the pH electrode in contact with the acid solution. • Rinse the 50-mL buret with a few milliliters of the 0.10 M sodium hydroxide solution. 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. • Turn on the magnetic stirrer.
Record Data 1. Arrange the displays to see the table of “pH and Volume”. 2. Click the Startbutton to begin recording data. The Start button changes to a Keep button. • Note: Do not add any NaOHtitrant 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 titrantto 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.
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. 13. If time permits, repeat the procedure. 14. Save your DataStudio file as your group members’ name to the location of Document>EDCP447>Chemistry folder.
Analyze • Write a lab report using the Workbook in the Data Studio. 1. Insert your graph of pH versus Volume. 2. 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. 3-1. Use the values from Table 1, calculate the number of moles of NaOHused. Record the number in Table 2. 3-2. 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. 4-1. Use the equation for the neutralization reaction to calculate the number of moles of HCl used. Record the number in Table 2. 4-2. 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.