Titration - 1 Experiment 7 Titration of an Antacid...

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1 Experiment 7: Titration of an Antacid Objective: In this experiment, you will standardize a solution of base using the analytical technique known as titration . Using this standardized solution, you will determine the acid neutralizing power of a commercially available antacid tablet. Introduction An understanding of the properties of acids and bases is an essential part of understanding chemical reactions. In aqueous solutions, a compound that produces H + ions upon dissolution is termed an acid . A compound that produces OH ions when dissolved in water is called a base . The reaction of an acid and base is a neutralization reaction, the products of which are a salt and water. In an aqueous solution, virtually all of the OH ions present will react with all of the H + ions which are present: H + ( aq ) + OH ( aq ) H 2 O ( l ) Because this reaction is essentially quantitative, it is possible to determine the concentration of an acid or base in an aqueous solution with high accuracy. When a solution of hydrochloric acid, HCl, is exactly neutralized with a solution of sodium hydroxide, NaOH, the number of moles of NaOH used will equal the number of moles of HCl originally present. The following relationship then holds true: moles NaOH = moles HCl (M NaOH )(V NaOH in liters) = (M HCl )(V HCl in liters) Eq. 1 where M = concentration in molarity and V= volume. If three of the above quantities are known, the fourth can be calculated. In order to determine when a solution has been exactly neutralized, an acid-base indicator is used which changes color in a certain pH range (pH is a scale used to measure acidity). This color change is termed the endpoint of the titration. Because the pH of a neutral solution is 7, an indicator that changes color near this pH should be used for an acid-base titration. Phenolphthalein indicator changes color in the range pH = 8.3 – 10.0 and can be used to determine when the correct amount of base has been added to an acidic solution to exactly neutralize it. Standardization of a Sodium Hydroxide Solution In order to determine the concentration of an acidic or basic solution, it is necessary to know the number of moles of acid or base which are required to neutralize it. This quantity can be calculated by accurately weighing a solid sample of an acid or a base, dissolving it in water and titrating this solution; that is, adding the solution of unknown concentration to it until the endpoint has been reached (see Tro, pp 163-165). It is difficult to accurately weigh sodium hydroxide since it is hygroscopic (absorbs water readily from air). A solution of NaOH is usually standardized using an acid known as a primary standard. A primary standard must satisfy the four following criteria: 1. Solid compound that is not hygroscopic and can be easily handled
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2 2. Is available in very pure form 3. Stable 4. Has a medium to high molecular weight For this experiment, a solution of NaOH, which has an approximate concentration
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Titration - 1 Experiment 7 Titration of an Antacid...

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