Chemistry 151 Last Updated Dec. 2013 Lab 10: The Neutralizing Ability of an Antacid (Titrations, Pt II) Introduction The active ingredient of many antacids is a base that neutralizes excess stomach acid, hydrochloric acid (HCl) in a simple acid-base reaction. For example, Phillips Milk of Magnesia takes its name from its active ingredient, magnesium hydroxide. When reacted with stomach acid, the reaction is Mg(OH) 2(s) + 2HCl(aq) MgCl 2(aq) + H 2O(l) Other antacids use calcium carbonate (CaCO 3) as the active ingredient, which neutralizes the acid by the following reaction. CaCO 3(s) + 2HCl(aq) CaCl 2(aq) + CO 2(g) + H 2O(l) In this lab, you will perform a titration to determine the acid-neutralizing capability of a commercial antacid containing calcium carbonate. Unlike the KHP used in the previous week titration, though, most antacid tablets have poor solubility in water (see equations above), which can affect the accuracy of your titration. So instead of titrating the antacid directly, you will first neutralize the antacid with an excess amount of hydrochloric acid then determine the amount of excess HCl present by titrating it with sodium hydroxide (NaOH). NaOH(aq) + HCl(aq) NaCl(aq) + H 2O(l) This technique is often referred to as a back-titration. Once you know the amount of excess HCl, you can determine the amount required to neutralize the antacid tablet. In this lab You will be using the same 1M NaOH solutions you standardized during the previous lab, so unless notified by your instructor you should use your calculated concentration from that lab. The HCl solutions have already been standardized for you. The indicator used in this lab will be bromophenol blue, which is yellow in acidic solution and blue in basic. Ideally the solution will turn green at endpoint (yellow + blue = green), but the green range is narrow enough that it can be difficult to see this color. You ll most likely see it jump straight from yellow to blue with your final drop of titrant, but this won t adversely affect your calculations. Before you perform the titration, however, we must first boil the solution to remove any dissolved CO 2. If not removed, the CO 2 can react with water to form carbonic acid (H 2CO 3), which can react with calcium chloride to precipitate calcium carbonate and form additional HCl. When this occurs, the trial becomes unusable. CO 2(aq) + H 2O(l) H 2CO 3(aq) H 2CO 3(aq) + CaCl 2(aq) CaCO 3(s) + 2HCl(aq)
Procedure Part I: Preparing the Buret 1. Attach a buret clamp to a ring stand. 2. Add 20 ml 1 M sodium hydroxide (which you prepared as part of the previous lab) to a buret and place the buret in the clamp. Inspect the stopper for leaks. If you see any, inform your instructor before continuing. 3. Slowly pour the sodium hydroxide into the sink, rotating the buret as you do in order to clean as much of the inside as possible. Repeat this step with an additional 20mL of sodium hydroxide. 4. Clamp the buret and fill it to the 0.0 ml mark with sodium hydroxide. If air bubbles form, try to remove as many as you can by gently tapping the side of the buret. Part II: Neutralizing the Antacid 1. Obtain an antacid tablets from your instructor measure its mass. 2. Place a tablet in an Erlenmeyer flask and add 75 ml water. 3. Using a volumetric pipet, transfer 40.00 ml of hydrochloric acid (approx. 0.6 M) to the flask and mix (the tablet may not completely dissolve). 4. Use a hot plate to boil the solution (vigorously) for 20 minutes. Part III: Performing the Back-titration 1. With the solution still warm, add 8 drops of bromophenol blue to the antacid solution. 2. Begin titrating sodium hydroxide to the solution. As soon as you see a green or blue color appear in the solution, turn off the valve. [Note: if you start to see a precipitate form, stop titrating immediately, as you re most likely forming calcium carbonate (see intro). Boil the solution for another ten minutes then continue with the titration.] 3. Swirl the flask to mix the solution then continue adding sodium hydroxide dropwise to flask. Continue swirling the flask as you add titrant until a green or blue color persists for 30 seconds. Repeat Parts II and III with a second tablet. Do not refill the buret unless it appears that you won t have enough to perform the next determination. Only perform a third trial if one of the first two becomes unusable because of excess CaCO 3 formation. Waste Disposal All waste can be poured down the sink with running water. Rinse you buret with water to remove any remaining traces of NaOH, which can dry up and clog the buret tip.
Name: Section: Data Standardized concentration of NaOH, M (from Lab 9) Standardized concentration of HCl, M (from label) Brand of antacid used Trial 1 2 3 Mass of tablet, g Volume of HCl added Total moles of HCl added Initial buret reading, ml Final buret reading, ml Volume NaOH titrated, ml Moles NaOH titrated Moles of HCl neutralized by NaOH Moles of HCl neutralized by antacid Moles of CaCO 3 in antacid Mass of CaCO 3 in antacid Mass percentage of CaCO 3 in tablet, g Average mass percent
Show your work for each of the following calculations from Trial #1. 1) Volume of NaOH titrated 2) Moles of NaOH titrated 3) Moles of HCl neutralized by NaOH 4) Moles of HCl neutralized by antacid 5) Moles of CaCO 3 in antacid 6) Mass of CaCO 3 in antacid 7) Mass percentage of CaCO 3 in tablet.
Name: Section: Post-lab Questions 1. If a student forms CaCO 3 from the presence of carbon dioxide, but continued with the trial anyway, how would this affect his/her calculations (too high, too low, or no effect)? Explain your answers. a) The volume of NaOH required for the back titration. b) The calculated moles of HCl neutralized by the antacid. 2. A student performed Part II as outlined in the procedure, but when he added the bromophenol blue the solution turned blue. a) Why is the solution blue? b) What can the student do to correct for this error and salvage this trial? How should the data sheet be modified to reflect this correction? 3. Some antacid tablets actually contain a mixture of calcium carbonate and magnesium hydroxide (though they usually have the same strength as those that only contain CaCO 3). If this had been the case for the antacid you tested, but you still assumed it only contained CaCO 3, how would this have affected each of the following (too high, too low, or no effect)? Explain. a) Moles of HCl neutralized by the antacid b) Mass percentage of CaCO 3 in tablet.
Name: Section: Pre-lab Questions 1. What is meant by a back-titration? Why is it being performed in this lab instead of a direct (normal) titration? 2. The bubbling you see at the start of your heating isn t the solution bubbling. What is it? 3. What color should the solution at the start of the titration performed in this lab? What color should it be when endpoint is reached? 4. How much 1.0 M NaOH would be required to completely neutralize 40.0 ml of 0.60 M HCl? 4. A 5.309 g antacid tablet, with CaCO 3 as the active ingredient, was mixed was 30.0 ml of 0.831 M HCl. It took 15.3 ml of 0.7034 M NaOH to neutralize the excess acid. a) Calculate the volume of HCl neutralized by the NaOH. b) Calculate the amount of HCl (in ml) neutralized by the antacid. c) Calculate the mass of CaCO 3 (in g) that reacted with HCl. d) Calculate the mass percent of CaCO 3 present in the antacid.