chem assignment 10

Experiment 10: EFFICIENCY OF ANTACIDS

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Purpose: The efficiencies of two different antacids to neutralize stomach acid are to be analyzed and compared. Introduction: Stomach acid (or gastric acid) is mainly an aqueous hydrochloric acid solution (approximately 0.1 M HCl). HCl is secreted by the stomach to help in the digestion of foods by breaking down large molecules into smaller ones to be absorbed by the small intestines. The human body can produce and secrete 2 to 3 liters of stomach acid per day. When we overindulge in food an excess of stomach acid is produced and secreted, resulting in the uncomfortable feeling of indigestion. If the excess acid were to make its way back up to the lower esophagus, we would experience heartburn. Heartburn, due to excess stomach acid, can be alleviated by neutralizing the acid with an antacid. For example, the active ingredient in Alka-Seltzer is sodium bicarbonate (NaHCO3). The reaction of sodium bicarbonate with stomach acid produces carbonic acid, which immediately decomposes to water and carbon dioxide:

NaHCO3 (s) + HCl (aq) ¾® H2CO3 (aq) + NaCl (aq) H2CO3 (aq) ¾® H2O (l) + CO2 (g)

The overall reaction is

NaHCO3 (s) + HCl (aq) ¾® H2O (l) + CO2 (g) + NaCl (aq).

The “fizz” produced when Alka-Seltzer is placed in acid is due to the carbon dioxide escaping out of the solution. Baking soda is pure sodium bicarbonate and works just like Alka-Seltzer as described above. It is much cheaper. However, one has to be careful with the dosage, which is roughly ½ teaspoon per 8-oz glass of water. Since it is not dispensed as a tablet, one could accidentally “overdose” on bicarbonate. Other antacids may have carbonates or hydroxides, such as calcium carbonate or aluminum hydroxide. Carbonates are similar to bicarbonates in that they produce water and carbon dioxide. Hydroxides react in a typical acid-base neutralization to form water and salt. An example of each is shown below:

CaCO3 (s) + 2 HCl (aq) ¾® H2O (l) + CO2 (g) + CaCl2 (aq) Al(OH)3 (s) + 3 HCl (aq) ¾® 3 H2O (l) + AlCl3 (aq)

In this experiment you will begin by preparing a simulated stomach acid solution. A higher concentration than what is normally found in the stomach (1.00 M instead of 0.1 M HCl) is used here so that we do not have to use unmanageably large volumes of solutions in the

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laboratory. Antacid is added to the acid, and the amount of acid not neutralized by the antacid is to be determined by measuring exactly how much NaOH is needed to finish the neutralization. An efficient antacid should produce a solution that requires little of the NaOH to complete the neutralization.

The method used for the analysis is called titration, which is a laboratory technique of determining the concentration of an unknown solution by measuring the exact volume of a solution of known concentration needed to completely react with the solute in the unknown solution. In this experiment, the leftover stomach acid is being titrated with 1.00 M NaOH. The volume of the NaOH solution is determined with the use of a buret. An indicator, bromophenol blue, is used, which turns from yellow (when in an acidic medium) to blue (when in a basic medium). Thus, when just enough (a stoichiometric amount) of NaOH has been added to neutralize the leftover stomach acid, the solution will quickly change to blue. This is called an endpoint, the point when an indicator changes color. In this reaction, the endpoint is also the equivalent point, the point when both reactants in a reaction have completely reacted, leaving no excess behind.

HCl (aq) + NaOH (aq) ¾® H2O (l) + NaCl (aq)

In this experiment, note that the balanced equation shows that one mole of HCl would require one mole of NaOH. Therefore, at and only at the exact point when the solution turns from yellow to blue, the number of moles of NaOH would equal to the number of moles of HCl. Since we are using the same concentrations (1.00 M) for both the HCl and NaOH, the volume of NaOH that is required to neutralize the leftover HCl after the antacid has done its job, is equal to the volume of HCl not neutralized by the antacid. CONCEPT OF MOLARITY Molarity is the number of moles of solute per liter of solution. Thus a solution labeled as 6.00 M HCl means 6.00 moles of HCl are in one liter of the solution.

Molarity =

Laboratory Techniques Used in This Experiment Pipeting Technique Pipets are accurately calibrated to deliver designated volumes of liquids. The proper technique of pipeting a sample involves filling the pipet above the calibration mark with the use of a pipet bulb or pipet pump. NEVER use your mouth to draw up the liquid. Hold your index finger over the top of the pipet to keep the liquid above the mark, wipe the outside of the pipet with a piece of Kimwipe, and then let the liquid slowly drain until the meniscus reaches the calibration line. Any drop hanging outside the tip of the pipet is removed by touching the tip of the pipet to the rim of the liquid container. The pipet is then shifted to the container into which you are to deliver the

Liters # moles #

calibration mark

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sample and is then permitted to drain without immersing it in the sample. The drop hanging OUTSIDE on the tip of the pipet after it has drained is removed by touching the tip to the inside wall of the container just above the liquid level. DO NOT BLOW OR SHAKE OUT THAT LAST DROP INSIDE THE TIP. The pipet has been calibrated with that last drop remaining in the tip. If you blow or shake that drop into the container, you would have transferred one drop too much. With the use of the wash bottle, direct a small stream of deionized water to the inside surface of the container to wash adhering drops of the sample to the bottom of the container. How to Record Buret Readings 1. The curved surface of a liquid is called a meniscus. Water has a meniscus that curves

down. It is important that you read the buret with the meniscus at eye level to avoid the problems of parallax. You know you are not at eye level if the marking on the buret on the backside is visible, as in Fig.11.1. If you are at eye level, the markings should be as shown in Fig. 11.2.

Fig. 11.1 Fig. 11.2 2. The scale on the buret goes from zero at the top to 50.00 mL at the bottom. It is easy to

forget and read it backwards. Read the burets to 2 decimal places. Remember you should be reading the number indicated by the bottom of the meniscus.

Buret A Buret B Buret C Buret D Buret Reading for Buret A = ______ Buret Reading for Buret C = ______ Buret Reading for Buret B = ______ Buret Reading for Buret D = ______

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PROCEDURE (Using a pen or pencil, record by hand all of your data and results on the Data Collection and Results Pages.) You will be using two antacids: baking soda and Tums. It might be necessary to repeat one or more of the trials if you were to miss the endpoint. The usual recommended dosage for Tums is to chew 2 to 4 tablets. With baking soda, it is suggested to be ½ teaspoon every 2 hours. In this experiment, we will be comparing the efficiency of 1 tablet of Tums and ¼ teaspoon of baking soda. Part I: Preparing the Upset Stomach

1. Obtain two 250-mL Erlenmeyer flasks from your drawer and label them as Tums and baking soda.

2. Label your 400-mL beaker as “waste” and set it aside. Obtain a clean and dry 250- mL beaker and label it as “HCl.” Obtain a clean and dry 100-mL beaker and label it as “NaOH.”

3. Pour 20 mL of 1.00 M HCl into the HCl beaker. Obtain a 25-mL volumetric pipet. Draw about a third of the HCl solution into the pipet and roll it around while holding the pipet horizontally as demonstrated by your instructor. This is to rinse the inside walls of the pipet. Let the solution drain into the “waste” beaker. Do not let it drain back into the HCl beaker.

4. Repeat the rinse two more times, each time letting the rinse solution drain into the “waste” beaker.

5. Discard any acid remaining in the 250-mL beaker into the “waste” beaker as well. 6. Obtain approximately 80 mL of fresh 1.00 M HCl in the same “HCl” beaker. 7. Following the directions given by your instructor, pipet exactly 25.0 mL of 1.00 M

HCl to each of the two flasks. Remember not to blow or shake out that last drop in the tip of the pipet. Record this volume to 3 significant figures.

8. Add 2 or 3 drops of bromophenol blue indicator to each flask. 9. With the wash bottle, wash down the inside walls of the flasks with deionized water

so that there are no acid drops hanging on the upper walls of the flasks. Part II: Adding the Antacid

10. Record the identity and amount of the active ingredient(s) of Tums and baking soda. 11. Obtain a mortar and pestle. If it looks dirty, just wipe it clean with paper towels.

DO NOT WASH OR GET IT WET IN ANYWAY. 12. Crush and grind one tablet of Tums with the mortar and pestle. 13. Using the balance, tare a weighing boat to zero. With your spoonula, transfer all of

the resulting fine powder to the weighing boat, and then record the mass using all decimal places shown on the balance. Then, carefully transfer the antacid from the weighing boat to the Erlenmeyer flask labeled “Tums”. (Do not spill any during the transfer.)

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14. Next, tare a clean (new) weighing boat to zero. Remove it from the balance pan, and add one small scoop of baking soda to it. Return the weighing boat to the balance pan. Continue removing the boat and adding a small amount of baking soda until the balance reads between 1.2 and 1.5 g. Record the mass to all the decimal places shown on the balance. (This is about ¼ teaspoon of baking soda.) Carefully transfer the antacid from the weighing boat to the Erlenmeyer flask labeled “baking soda”.

15. You have now partially neutralized your two “upset stomachs” with antacid samples. 16. In Part III, you will determine how much of the acid did not get neutralized.

Part III: Completing the Neutralization

17. PREPARING THE BURET: a. Obtain a 50.00-mL buret and set it up on a ring stand with a buret clamp as

shown in the Figure 10.3 below. Position a magnetic stir-plate so that the buret is directly over the center of the stir-plate.

b. Into the NaOH beaker, pour about 10 mL of the 1.00 M NaOH. c. Make sure the stopcock is in the CLOSED position and transfer about half of the

contents of the beaker into the buret. d. Hold the buret in a near-horizontal position with the opening over the “waste”

beaker. e. Rotate the buret so that the inside surface of the buret is coated with the NaOH

solution while it is being drained SLOWLY into the “waste” beaker. f. Repeat Steps (c) through (e) with the other half of the NaOH solution in the

NaOH beaker.

stopcock

magnetic stir-plate

Figure 10.3

buret clamp

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g. Pour 50 mL of 1.00 M NaOH into the “NaOH” beaker. This should be enough for both trials.

h. Hold the buret at a slight angle. Make sure the stopcock of the buret is closed, then pour all of the NaOH solution into the buret.

i. Place your “waste” beaker under the buret, turn the stopcock to the fully open position and allow about 5 mL of NaOH solution to drain out of it. This should get rid of all of the air pocket in the buret tip. It is essential that there are absolutely no air bubbles remaining in the tip. It is usually difficult for beginning chemistry students to recognize the presence of air bubbles in the tip. Ask for assistance if you are not sure.

18. Hold the buret at eye level, and with the help of the volume-reading card you had prepared in an earlier experiment, read and record the buret reading to two decimal places (0.01 mL) under “Initial Buret Reading.” Place the buret back on the buret clamp.

19. Obtain ONE magnetic stir-bar from your instructor. Rinse it with deionized water and the carefully slide it into your Tums Erlenmeyer flask, and place the flask under the tip of the buret. The flask should be close to the center of the magnetic stir-plate. The tip of the buret should be inside the flask, slightly below the lip of the flask so that no NaOH solution can splash out during the subsequent steps.

20. Place a piece of white paper under the flask to help you detect more readily the change in color during the titration.

21. Turn on the magnetic stirrer and slowly increase the speed to medium speed. Be sure you are using the STIR knob and not the HEAT knob. You do NOT want to heat your solution! You also do not want to stir it at a speed such that droplets may splash out of the flask.

22. Adjust the stopcock so that the NaOH solution comes out very slowly, drop by drop. It should be slow enough that when necessary, you can turn off the stopcock immediately before the next drop comes out.

The bromophenol blue indicator is yellow when the solution is acidic and it turns blue when the solution is basic. You want to add just enough NaOH to neutralize all of the acid. Initially, when the first pale blue begins to appear, it will quickly disappear as you stir the contents of the flask. As you near the endpoint, you will have to stir longer before the blue disappears, and you should slow down the drops. You have reached the endpoint when the solution stays a pale blue even after 30 seconds of stirring. The endpoint is very sharp. It takes only one drop (or half a drop if you are careful enough) to bring it from yellow to pale blue. You must stop the addition immediately. If you were to allow extra drops to fall into the solution you would have passed the endpoint and your data would be incorrect.

Note: Be sure that at no point should the NaOH level go below the graduated portion of the buret (below 50.00 mL). If you think you are going to run out of NaOH in the buret, notify your instructor who will decide what you should do.

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23. Continue adding the NaOH solution dropwise until a pale blue color persists after 30 seconds of stirring. Do not overshoot the endpoint.

24. After you have reached the endpoint, hold the buret at eye level, read and record the buret reading (again to 0.01 mL) under “Final Buret Reading.”

25. From the difference in the two buret readings you can calculate the volume of 1.00 M NaOH you have used to neutralize the remaining “stomach acid” that the antacid did not neutralize. CALCULATE THIS IMMEDIATELY!

26. The solution in the flask is essentially neutral and can be poured down the drain. Be careful the stir-bar does not go down the drain also! Rinse the stir-bar under running tap water and then rinse it with deionized water before you use it again for the next trial.

27. Repeat Steps 19 through 26 for the baking soda trial. If your buret is not leaking, the “Final Buret Reading” of the previous trial would become the “Initial Buret Reading” of the next trial. You probably will not need to refill the buret at any point unless you have made a mistake and have to repeat a trial.

28. FINAL CLEAN UP: The contents of the flasks can be discarded down the drain. The white residue on the walls of the flasks MUST be removed. Rinse the flasks out with a small amount of HCl solution provided IN THE HOOD. The rinse should go into the designated waste container. The excess HCl or NaOH from the beaker or buret should be discarded in the same waste container. Return the stir-bar to your instructor. The mortar and pestles should be washed and wiped dry with paper towels. All labels should be removed from your beakers and flasks.

The buret (with the stopcock OPEN) and the pipet should be rinsed with hot tap water and then GENTLY placed in the special buret container marked “DIRTY” with the tips pointing up.

*Answers to example buret readings in introduction (remember that the last digit is the uncertain digit - it is the only digit that can be slightly different from person to person): A) 43.65 mL B) 41.05 mL C) 0.25 mL D) 18.15 mL