• Calculate K a1 , K a2 , p K a1 , and p K a2 of phosphoric acid. MATERIALS LabQuest various cola soft drinks, decarbonated LabQuest App distilled water Vernier pH Sensor ring stand 50 mL buret utility clamp 100 mL graduated cylinder magnetic stirrer 250 mL beaker stirring bar 0.050 M NaOH buret clamp PROCEDURE 1. Obtain and wear goggles. 2. Using a graduated cylinder, measure out 40 mL of a decarbonated cola beverage and 60 mL of distilled water into a 250 mL beaker. CAUTION: Do not eat or drink in the laboratory. 3. Place the beaker on a magnetic stirrer and add a stirring bar. 4. Connect the pH Sensor to LabQuest and choose New from the File menu. 5. Use a utility clamp to suspend a pH Sensor on a ring stand as shown in Figure 1. Position the pH Sensor in the soda and adjust its position so it is not struck by the stirring bar. 6. Obtain a 50 mL buret and rinse the buret with a few mL of the 0.050 M NaOH solution. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing. Dispose of the rinse solution into the waste container. Use a buret clamp to attach the buret to the ring stand as shown in Figure 1. Fill the buret a little above the 0.00 mL level of the buret with 0.050 M NaOH solution. Drain a small amount of NaOH solution so it fills the buret tip and leaves the NaOH at the 0.00 mL level of the buret. Record the precise concentration of the NaOH solution in your data table. Figure 1
7. Set up the data-collection mode. a. On the Meter screen, tap Mode. Change the data-collection mode to Events with Entry. b.Enter the Name (Volume) and Units (mL). Select OK. 8. You are now ready to perform the titration. This process goes faster if one person manipulates and reads the buret while another person enters volumes. a. Start data collection►. b. WARNING: Do not hit stop until you take your final measurement or you will have to start the titration over again!!!!!!!! c. Before you have added any NaOH solution, tap Keep and enter 0 as the buret volume in mL. Select OK to store the first data pair for this experiment. d. Add a small amount of NaOH up to 0.5 mL. When the pH stabilizes, tap Keep and enter the current buret reading. Select OK. e. Continue to add 0.5 mL increments, entering the buret level after each increment. When the pH has leveled off (near pH 10), stop data collection . 9. Examine the data on the graph of pH vs. volume to find the equivalence point —that is, the 0.5 mL volume increment that resulted in the largest increase in pH. As you tap each data point (or use the ► or ◄ keys on LabQuest), the pH and volume values are displayed. Go to the region of the graph with the large increase in pH. Find the NaOH volume (in mL) just before this jump. Record this value in the data table. Then record the NaOH volume after the 0.5 mL addition producing the largest pH increase. Remember that there are two large pH increase because you are going through two equivalence points.
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- Spring '12
- pH, Sodium hydroxide