POGIL LAB EXERCISE 15 HOW DO YOU STANDARDIZE AN ACID AND BASE?

POGIL LAB EXERCISE 15 HOW DO YOU STANDARDIZE AN ACID AND BASE? POGIL LAB 15 Page 1 of 10 Each member should assume his or her role at this time. The new manager takes charge of the POGIL folder and hands out the GRF and RRF to the appropriate members. The new recorder should record the names of the group members on the new GRF and the new recorder should record the start time for the lab. Table 1. Group Member Role Assignments GROUP TYPE -> GROUPS OF THREE GROUPS OF FOUR MEMBER NO. -> 1 2 3 1 2 3 4 Manager + + Reporter + + Recorder + + Reflector + + Technician + Encourager + + SFUC * + * + Each member should assume his or her role at this time. The new manager takes charge of the POGIL folder and hands out the GRF and RRF to the appropriate members. The new recorder should record the names of the group members on the new GRF and the new recorder should record the start time for the lab. OBSERVATION I. One of the most common analytical process used in chemistry is the process of titration. Titration is the process by which a solution is added in a controlled manner to another solution to a specific end point. The measuring device used in analytical labs for this purpose is called a burette. In today s experiment the NaOH solution you prepared will be placed in the burette. The burette is the most accurate tool in our lab of delivering volumes continuously from 0.10 ml to 50.0 ml that is accurate to the nearest.005 ml. This means that all volumes read from the burette has three decimal places; the last place is the least accurate because it is estimated by inspection. The specific end point of the titration is detected by an indicator that changes color at or very near the end point. In today s lab the indicator is phenolphthalein. Phenolphthalein is clear at ph values below seven and becomes pink at ph values above seven the darker the color the higher the ph. The end point is when the first stable very light pink color is formed. The ph at this point is seven within our ability to measure it. + 5 MIN

POGIL LAB 15 Page 2 of 10 1. Why is the burette used to measure the volumes rather than a graduated cylinder? 2. What is an indicator and what role does it play in the titration process? 3. What is the name of the indicator used in this experiment? 4. How does this indicator detect the end point of an acid/base titration? 5. What solution does the burette hold? Submit your responses to Items 1-5 to instructor for validation. OBSERVATION II. The usual titration experimental set-up for this experiment is pictured and diagramed below in Figure I. In addition to the burette other common laboratory Figure I. Experimental Set-up for Todays Lab tools are used to increase the accuracy of the experiment: A ring stand is used as the central point in the setup. Attached to the ring stand is the burette (butterfly) clamp. This clamp is designed to hold the burette in an exact horizontal position. Care should be taken when the burette is placed in the clamp so that the burette lies exactly in the middle grove of both burette clamps on one of the arms of the clamp as pictured in the diagram in Figure 1. + 10 MIN

POGIL LAB 15 Page 3 of 10 OBSERVATION II. The titration vessel used in today s lab will be a 125 ml Erlenmeyer flask. This flask will hold the acid and the phenolphthalein solutions. A Teflon magnetic stirring bar is also placed in the flask. Care should be taken to make sure that the stirring bas is in the middle of the flask and covered by the acid solution. The station is completed by placing the hot plate/magnetic stirrer in position. This devise provides the moving magnetic energy to activate the stirring bar. 6. What is the role of the ring stand in the set-up? 7. What is the role of the magnetic stirring bar? 8. What is the role of the burette clamp? 9. What is the role of the magnetic stirrer? Submit your responses to Items 6-9 to instructor for validation. OBSERVATION III. Bases must be continually standardized because carbon dioxide in the air reacts with the water in the solution to yield carbonic acid (H2CO3) and this acid reacts with the base reducing the amount of base in solution. The fastest and easiest method of standardizing a base is using a solid acid that ionizes to produce just one hydrogen ion. A solid acid is desirable because it can be measured by weight very accurately and thus the theoretical amount of hydrogen ion can be calculated precisely. The acid chosen most often for this job is potassium acid phthalate (POP; FM = 204.1 g). The use of the solid acid involves the two basic definitions of a mole we have studied below defined in Equations 1-3: EQ1: M = n/l This equation can be rearranged into another useful form shown in Equation 2. EQ2: n = (M)(L) EQ3: n = mass (g) Formula mass (F) + 20 MIN

Since both Equations 2 and 3 are equal to moles they are equal to each other (Equation 4). Therefore POGIL LAB 15 Page 4 of 10 EQ4: mass (g)/f = (M)(L) or mass (g) = (M)(L)(F) 10. You will be using 500 mg (0.500 g) of is potassium acid phthalate to standardize your base. Assume that you were extremely accurate in your preparation and the concentration of your solution is 0.200 M. What volume of your base (in ml) would be required to neutralize the 500 mg of the acid Submit your response to Items 10 to instructor for validation. OBSERVATION IV: Standardization of the Sodium Hydroxide Solution. Standardize the solution using the procedure described below. All references to rows in this section refer to Table 2. Table 1 is also reprinted in part on the GRF and must be completed appropriately by the recorder before the group leaves the lab. 11. Two experimental stations without the burette should be setup at each group having 4 members. Groups of three should set up one station. The experimental setup at each station should be as described in Observations I and II and seen in Figure 1. The following steps should be carried out at each station. 12. Rinse a clean 250 ml beaker three times with approximately 10 ml of the unstandardized NaOH solution. Add 75 ml of the solution to the beaker. (Use the volume scale on the beaker.) 13. Preparation of burette for titration. a. Obtain a 50 ml burette from the wall storage rack, remove the stopper from the top and set it aside. b. Turn burette upside down over the sink and open the stopcock at the top to let the water drain out faster. When the burette is empty, close the stopcock and place the burette in the burette clamp. + 30 MIN

POGIL LAB 15 Page 5 of 10 c. Add 10 ml portion of distilled water to the burette; empty burette as described in Step b. d. Repeat Step c. e. Add 10 ml of the 0.2 M NaOH solution to the burette; empty burette as described in Step b. f. Repeat Step e two times. g. Fill the burette several centimeters above the 0.00 ml line with the NaOH solution. h. Drain the solution from the burette into a beaker so that the meniscus rests on the 0.00 ml mark. (If you miss the zero line shoot for the 1.00 mark.) Be sure that there are no air bubbles in the tip of the burette; if there are any bubbles, add more sodium hydroxide solution and drain until the air bubbles are gone. 14. Enter starting volume (probably 0.00 for first sample) in the appropriate column of Row 3. 15. Add about 50 ml of distilled water (using measure on flask) to the 125 ml Erlenmeyer flask and then add the magnetic stirring bar. 16. Add 2 to 3 drops of the phenolphthalein solution to the flask. (If the flask changes color you have done something wrong and need consult the instructor.) 17. Accurately measure (i.e. to as many decimal places as the balance has) in a plastic weighing dish 450-550 mg of potassium hydrogen (acid) phthalate. Record the exact mass in appropriate column of Row 1. Quantitatively transfer the PAP to the flask. 18. Place the flask onto the magnetic stirrer (Make sure the heater is turned off.). Turn the stirrer speed to between 2 and 4 to ensure adequate stirring without sloshing the solution out. Make sure you do not accidentally turn on the heat as well, as this will affect your data. Stir gently until all the PAP has dissolved. 19. The target volume for your titration is the volume calculated in Step 10. At the start you can add 2 ml at a time until you have run 80% of the calculated value. Then make a few 0.5 ml additions and then finally a drop at a time is added as you approach the calculated value until the first permanent pink color appears (i.e. color remains a light pink for at least 15 seconds after swirling). 20. Read and record the volume of base added, to the nearest 0.01 ml, in the appropriate column of Row 4. 21. If this is the end of you third determination go to Step 24; otherwise, drain the burette to the next whole milliliter mark. + 45 MIN

Table 2. Standardization of Your 0.2 M NaOH Solution Standardization of NaOH solution A Trial 1 B Trial 2 POGIL LAB 15 Page 6 of 10 C Trial 3 1. Mass of potassium acid phthalate 2. Estimate of volume NaOH needed: 3. Initial burette reading (ml of NaOH) 4. Final burette reading (ml of NaOH) 5. Milliliters of NaOH used (4-3) 6. Liters of NaOH solution used 7. Molarity of NaOH solution 8. Average Molarity of NaOH solution 9. % Accuracy 10. % Precision CALCULATIONS: 22. Discard the solution in the flask in the sink after the titrations are completed. Rinse out the flask with small quantities (about 5 ml) of distilled water. You do not have to dry the inside of the flask. 23. Repeat Steps 14-22 two more times. 24. The recorder should fill out her/his report form while the rest of the members do the following: Empty the beaker containing your NaOH solution into the sink and flush it down with a tap water. Set the beaker aside. Turn off the magnetic stirrer. When all is completed continue by processing with Observation V. + 60 MIN

POGIL LAB 15 Page 7 of 10 OBSERVATION V. We are now able to use our standardize base to standardize an acid solution. In Experiment 2 you will be asked to determine the exact concentration of an HCl solution that is labeled 0.5 M. This solution is much too concentrated to titrate directly with your NaOH solution so we will titrate exactly 5.0 ml of the HCl solution. The most accurate method of measuring the quantity of concentrated acid needed for this preparation is by using a volumetric pipette to deliver the required volume. This is a skill that needs to be demonstrated to you and practiced before the preparation of the solution. This will be accomplished by following directions in Items 2-7. 25. Obtain a dry 5.0 ml volumetric pipette from the supply table as you obtain your lab kits. 26. Examine the markings on the pipette: Copy these in the space below. 27. What do these markings mean? 28. Locate an empty amber storage bottle on the supply table and a clean 100 ml beaker from your lab kit; place about 50 ml distilled water in the beaker. 29. Notify instructor for demonstration of quantitative pipetting technique. 30. Each member of the group should practice pipetting at least one or, time permitting, two 5 ml samples from the beaker to the storage bottle according to the following procedure: a. The manager should be the first to practice. b. Other group members should observe the pipetter making sure that she/he does as the instructor demonstrated. c. The pipetter then passes the implements to the next person in the group. e. Steps 7a-7c are repeated until each person in the group has had a turn. d. When all members of the group have had a turn, screw the cap on the storage bottle tightly and shake to rinse the bottle. The pour the water into the sink. Set the bottle aside to be used again. f. Repeat Steps 7a-7d. g. Return the pipette to the used receptacle on the supply table, screw the cap on the storage bottle and set the bottle aside for use in Item 11. h. Invert the bottle and shake as much water from inside it out into the sink. Return to supply table and invert on paper towels set out for that purpose. +70 MIN

POGIL LAB 15 Page 8 of 10 OBSERVATION VI. The calculations involved in the standardization process involves a familiar relationship illustrated by Equations 5-? Below: EQ5. At neutralization the amount of acid = to amount of base; leads to EQ6. CaVa = CbVb where C = concentration and V is the volume of the acid and base respectively. 31. Suppose the concentration of the acid solution is exactly 0.500 M. What volume of the base is required to neutralize 5.00 ml of the acid solution? Present your response to the instructor for validation. Standardization of an Acid Solution. Each experimental station should carry out the following procedure. 32. Obtain a bottle of the 0.5 M HCl solution and a dry 5.0 ml volumetric pipette from the supply table. 33. Place approximately 50 ml of distilled water, 2 to 3 drops of the phenolphthalein solution and the magnetic stirring bar in the 125 ml Erlenmeyer flask. 34. Quantitatively transfer 5.00 ml of the 0.5 M HCl solution to the Erlenmeyer flask using the 5 ml volumetric pipette. (A different group member should do the pipetting for each of the samples.) 35. Locate you re the beaker that you used to hold the large amount of your NaOH solution and rinse it a couple of times with about 10 ml of the NaOH solution. 36. Add approximately 75 ml of the base to the beaker. 37. Add the base from the beaker to the burette to a couple of centimeters above the zero line; drain into waste beaker until the meniscus is exactly on the zero line. 38. Repeat Steps 14-23 entering relevant data in Table 3. +80 MIN

Table 3. Standardization of the Acid Standardization of NaOH solution A Trial 1 B Trial 2 POGIL LAB 15 Page 9 of 10 C Trial 3 1. Volume of 0.5 M HCl Solution 5.00 ml 5.00 ml 5.00 ml 2. Estimate of volume NaOH needed: 3. Initial burette reading (ml of NaOH) 4. Final burette reading (ml of NaOH) 5. Milliliters of NaOH used (4-3) 7. Molarity of NaOH solution 8. Molarity of HCl Solution 9. Average Molarity of NaOH solution 10. % Accuracy 11. % Precision CALCULATIONS: 39. The recorder should begin to transfer the collected data for the experiment their report form. When the recorder is finished, she/he should help with Step 40. 40. The cleanup should include the following items: a. All glassware should be rinsed with tap water and empties of fluid as much as possible. b. The burette should be emptied, rinsed with tap water and then with distilled water several times. The burette should be filled with distilled water, drained until all the air is out of the tip, stoppered and returned to the storage rack. c. All supplies should be returned to the supply table. d. All items in the lab kit should be returned to their storage places. +100 MIN

POGIL LAB 15 Page 10 of 10 EXERCISE END. Managers should collect the GRF and RRF, paper clip them together, and place in the back of the left pocket of the folder. The folder should be closed and left on the table. REPORTER, NOTIFY INSTRUCTOR WHEN FINISHED. WAIT FOR INSTRUCTIONS. MINILAB REPORT. A report of one or two paragraphs plus data tables should be prepared that indicates the concentration of the NaOH and the acid solution. The report should also state the accuracy and precision of each process and interpret these results as to their meaning for how well the lab was done and the validity of the findings.