Name Lab Section Chemistry 132 Lab 11 How Effective is Your Antacid? Prelaboratory Exercise 1. How many grams of NaOH will you need to make 250 of 0.5 M NaOH solution? 2. What is the purpose of the first, quick titration you will do? How is it different from the three subsequent titrations you will do? Laboratory Summary 1. Prepare 250 of NaOH solution with a concentration of about 0.5 M. 2. Standardize the sodium hydroxide solution with hydrochloric acid solution (four titrations). 3. React an antacid with the hydrochloric acid solution to dissolve it, and use your standardized sodium hydroxide solution to determine the ability of the antacid to neutralize acid. Introduction This experiment consists of three parts. In the first part, you will prepare a solution of sodium hydroxide (NaOH), a strong base. As you may recall, a solution is a homogeneous mixture. In aqueous solutions, water is called the solvent and the other compound is called the solute. One common way of describing how much solute is dissolved in a solution is molarity or the molar concentration. Molarity is defined as s of solute divided by the volume of solution in liters, and the equation is as follows: s of solute Molarity = (1) liters of solution Molarity is abbreviated as M, and has units of s/liter. Hence a 1 M hydrochloric acid (HCl) solution contains one of HCl per 1 L solution. In this experiment you will have to make 250 of an NaOH solution whose molarity is approximately 0.5 M. You need to calculate the mass of sodium hydroxide such that when it is diluted to 250 it has a molar concentration of 0.5 M. Solid sodium hydroxide will absorb moisture from the air, so it is not possible to measure the mass of NaOH with high accuracy. It is therefore difficult to prepare sodium hydroxide solutions of precisely known concentration. Instead, in part 2 of the experiment, you will carefully add some of your NaOH solution using a burette to an HCl solution whose concentration is known precisely using a procedure called a titration. A solution whose concentration is known precisely is called a standardized solution. The reaction between the NaOH and HCl is given by: NaOH (aq) + HCl(aq) H 2 O (l) + NaCl (aq) (2) We will be able to determine the unknown concentration of NaOH because a known quantity of HCl will be the limiting reagent. It will be completely consumed by an equivalent amount of NaOH. How will we know when we have added the same amount of NaOH as the amount of HCl initially present? We will add a chemical compound that changes color depending on whether it is in an acidic or basic solution. This compound is called an indicator and the indicator we are going to use is phenolphthalein. An acid-base indicator is a material that changes color when the ph of a solution changes from acid to base or from base to acid. Phenolphthalein is colorless in acidic solutions but is pink in basic solutions. Hence we will be able to tell when we have added a slight excess of base (we have consumed all of the HCl) when the color change is observed. A total of four titrations will be performed. 1
The third part of the experiment is to determine the amount of acid that an antacid can neutralize. Although antacids do not readily dissolve in water, they react with acids and the reaction products will dissolve. In order to determine the neutralizing power of the antacids, a known excess volume of hydrochloric acid will be added to the antacid. The amount of excess acid can then be determined by titrating the solution with the standardized base in order to find the amount of acid that was neutralized by the antacid. Experimental 1. Preparation of the ~0.5 M NaOH Solution The first step of the experiment is the preparation of 250 of NaOH solution with a concentration near 0.5 M. In order to prepare the solution, it is necessary to calculate the mass of solid NaOH that must be added to 250 of water. This calculation employs the following formula: mass of NaOH in grams = V NaOH M NaOH MM NaOH (3) where V NaOH = volume of NaOH solution in liters, M NaOH = molarity of NaOH solution, and MM NaOH = molar mass of NaOH in g/mol. Fill in the information on your data sheet and calculate the mass of NaOH required. Check the result with your instructor before beginning with the solution preparation. Volume of NaOH solution Molarity of NaOH solution Molar Mass of NaOH Mass of NaOH needed L M g/mol g After your calculation has been checked by your instructor, weigh out the mass of NaOH required into a 250 flask. CAUTION: Solid sodium hydroxide is a corrosive material that may cause serious skin and eye damage. Wear your safety glasses at all times. Add 250 of water to the solution, stopper, and mix well. Label the flask with the approximate concentration, the solute (NaOH), and your name. 2. Standardization of NaOH solution with HCl solution You will be provided with an HCl solution whose concentration is 0.500 M. Obtain approximately 220 of this solution in a clean, labeled 250 beaker. Use a graduated cylinder to dispense exactly 25.0 of this solution into four labeled 125 Erlenmeyer flasks. Add four drops of phenolphthalein indicator to each flask. In order to perform the titration, you will use a piece of glassware called a burette (Figure 1). First, add about 10 of NaOH solution to the burette and allow it to drain out, in order to rinse the burette. With the stopcock closed, fill the burette to near the top of the graduated region with the NaOH solution. (You do not have to hit the 0.00 mark exactly.) 2
Figure 1. Titration apparatus. A quick titration is first done in order to estimate the volume of NaOH needed to exactly neutralize the acid. Record the initial volume of NaOH solution in the burette (to the hundredths place). Add 20 of NaOH solution from the burette to Erlenmeyer flask #1, while swirling. The solution should appear pink momentarily, and then return to colorless with swirling. Next add NaOH to the flask 1 at a time until the solution stays pink. Record the final burette reading to the hundredths place. Subtraction of the final burette reading from the initial burette reading gives you the total volume of NaOH added. Three careful titrations will be performed in order to find the exact concentration of the NaOH. For these titriations, the volume of NaOH you can add quickly without going past the endpoint is 2 less than the volume that caused a color change in titration #1. For example, if your solution turned color after addition of 27, then 25 NaOH solution can be quickly run into to the flask. (Any additional NaOH will be added very slowly.) Fill in the data from quick titration #1 on your data sheet. Titration #1 approximate endpoint: Initial burette reading Final burette reading Volume to go past the endpoint Volume you can add quickly without exceeding the endpoint 3
Now you will perform the three careful titrations. Refill the burette with sodium hydroxide solution, record the initial burette reading to hundredths place in Table I below, and add the amount of NaOH you can add quickly without exceeding the endpoint to Erlenmeyer flask #2. Then slowly and carefully add NaOH solution one drop at a time with swirling, until the proper endpoint is reached. At the endpoint, the solution should turn faint pink, and this color should last for about 30 seconds then fade. (If your solution is a bright pink color that persists, you have gone past the endpoint.) At the endpoint of the titration, the added base will exactly neutralize the acid that was in the flask. Once you reach the endpoint, record the final burette reading (to hundredths place) in the table under Titration #2. Repeat this procedure for Erlenmeyer flasks #3 and #4. Table I Exact Titration Data Use Data Sheet Titration #2 (flask #2) Titration #3 (flask #3) Titration #4 (flask #4) Initial burette reading Final burette reading Volume of NaOH Molarity of HCl solution 0.500 M 0.500 M 0.500 M Molarity of NaOH solution M M M Average NaOH Molarity from three titrations M (transfer this number to Table II) The concentration of NaOH (M NaOH ) is determined for each titration from the following equation: M NaOH = M HCl V HCl 1 V NaOH (4) where V HCl is the volume of standardized HCl solution, M HCl is the molarity of the standardized HCl solution, and V NaOH is the volume of NaOH solution added. Perform this calculation three times and calculate the average molarity of the NaOH solution. 1. Why do you have to perform this titration three times? 3. Rolaids versus Tums Both Rolaids and Tums neutralize excess stomach acid (HCl) and both make various advertising claims. Let's find out which is more effective. Although antacids do not readily dissolve in water, they react with acids and the reaction products will dissolve. In order to determine the neutralizing power of the antacids, a known excess volume of hydrochloric acid will be added to the antacid. The amount of excess acid can then be determined by titrating the solution with the standardized base. Calculation of the s of acid neutralized per 4
antacid tablet (mol ant ) is achieved by use of the following equation, which simply states that at the endpoint the s of acid are equal to the s of base: (V HCl M HCl ) = (V NaOH M NaOH ) + mol ant (5) where V HCl is the volume of HCl solution added in liters, M HCl is the molarity of the HCl solution used in the previous portion of the experiment, V NaOH is the volume of NaOH solution added in liters, and M NaOH is the molarity of the NaOH solution standardized in the previous portion of the experiment. Equation (5) is rearranged to solve for the s of antacid which gives: mol ant = (V HCl M HCl ) (V NaOH M NaOH ) (6) Procedure. Separately, crush a tablet of each kind using the mortar and pestle designated for each antacid. Weigh the Rolaids and Tums powders and place each in a separate, clean 125 flask. Use your graduated cylinder to add exactly 50.0 of 0.500 M HCl to each flask and stir until the reaction has stopped. Add 4 or 5 drops of phenolphthalein indicator. Titrate with your standardized NaOH solution from the previous section and calculate the number of s of acid neutralized by each of the tablets using the equation above. Record results in Table II of your data sheet. Table II Tums Rolaids mass of powder g G Initial burette reading Final burette reading Volume of NaOH Volume of NaOH L L Molarity of NaOH (average from table in section #2) M M Molarity of HCl solution 0.500 M 0.500 M Volume of HCl 0.0500 L 0.0500 L Moles of antacid per tablet (mol ant ) tablet tablet Moles of antacid per gram of tablet g g 5
Questions - answer on-line 1. Which antacid is more effective per tablet? 2. Which antacid is more effective per gram of tablet? 6