Antacid_Procedure - Santa Monica College Chemistry 11...

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Santa Monica College Chemistry 11 Evaluating the Cost-Effectiveness of Antacids Objectives In this exercise, two commercially available antacids are evaluated and compared by: determining the number of moles of H + neutralized per gram of each antacid. calculating the cost effectiveness of each antacid. Background Acid indigestion is a common ailment caused by the overproduction of stomach acid, HCl. Over-the-counter antacids provide some relief from the symptoms of acid indigestion. They are generally made up of some mixture of weak bases such as Mg(OH) 2 , Al(OH) 3 , and CaCO 3 that can react with HCl as shown in these net ionic equations: H + ( aq ) + OH ( aq ) H 2 O ( l ) 2 H + ( aq ) + CO 3 2– ( aq ) H 2 O ( l ) + CO 2 ( g ) In this exercise, the method of titration will be used to determine the number of moles of H + neutralized per gram of antacid. In the “back-titration,” a portion of antacid will be mixed with an excess of HCl. The H + that has not reacted with the antacid is then titrated with standardized NaOH in the presence of the indicator bromophenol blue to a blue end point. The end point , is defined as the volume of OH needed to see a color change. Because only the tiniest excess of OH over H + can cause the color change of an indicator, the end point is a close approximation of the equivalence point (note that the difference between the end point and the equivalence point is known as the titration error ). At the equivalence point, the number of moles of OH added is equal to the number of moles of excess H + that had not been neutralized by the antacid. By knowing the total moles of HCl added, the number of moles of H + neutralized by the antacid can thus be calculated.

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