Exp16 - Experiment 16 IDENTIFICATION OF AN UNKNOWN ACID I....

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Experiment 16 IDENTIFICATION OF AN UNKNOWN ACID I. Learning Objectives… ± To determine the identity of an unknown acid by titration analysis. II. Background Information In this experiment, an acid-base titration will be performed in order to determine the dissociation constants and molar mass of an unknown, H n A. The study of titration curves is worthwhile since a great deal of acid-base chemistry can be assimilated in an organized, compact format. This information can then be applied to any system in which neutralization is taking place. Monoprotic and Polyprotic Acids and Bases In addition to being classified on the basis of their degree of dissociation (strong and weak), acids and bases are also classified as to the number of equivalents of acid or base that can be supplied per mole. ± + ± + + ² + ² OH Na NaOH Cl H HCl Both HCl and NaOH are classified as monoprotic species, both only capable of supplying one mole of hydroxide or hydrogen ion. Well known diprotic examples include the amino acids (shown as Zwitterions with both a positive and negative charge) and carbonic acid: CH + H 3 N C R - O O O H CO OH HCO O H HCO OH CO H 2 2 3 3 2 3 3 2 + ± + + ± + ² ² ² ² ²
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16-2 This general trend continues (triprotic, tetraprotic, etc.), with large compounds such as enzymes having 30 or more equivalents per mole. Titration Curves A titration curve is a graphical description of the solution pH as a function of the volume of added titrant. The utility of this approach is that the total curve can be divided into regions, each representing a different stage (reaction) involved in the titration process. Since neutralization is the same whether it is taking place in a titration vessel, in a cell, or on Mars, consideration of titration curve theory is an excellent approach to any type of problem involving acid-base equilibrium. The regions of a titration curve are usually defined as follows: Region Description 1 before the addition of any titrant 2 after titrant addition, but before the equivalence point 3 at the equivalence point (mol Acid = mol Base) 4 after the equivalence point
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16-3 Titration of a monoprotic strong acid with base This class of titrations is the simplest in terms of titration curve theory. Since, by definition, the titrand is completely ionized, the entire course of the titration is described by one equilibrium expression. O H OH H 2 ± + ² + The titration curve for this case is constructed as follows: Region 1: Since the acid is strong, the pH is simply calculated by determining the H + resulting from the presence of the strong acid. Region 2: Since the base titrant reacts completely with the acid present according to the above equation, the pH is calculated by determining the H + that remains after partial neutralization.
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Exp16 - Experiment 16 IDENTIFICATION OF AN UNKNOWN ACID I....

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