Experiment 6

Experiment 6 - EXPERIMENT 6 Acid-Base Titrations EXPERIMENT...

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EXPERIMENT 6 Acid-Base Titrations 47 E XPERIMENT 6 Acid-Base Titrations 6.1 Purpose In experiment 6, concentrations of various acidic solutions, as well as a value for the acid ionization constant K a of a weak acid will be determined. 6.2 Background Titration is a general class of experiments where a known property of one solution is used to infer an unknown property of another solution. In a titration, a titrant is added in controlled fashion through a burette to a solution of unknown properties. In acid-base chemistry, titration is often used to determine the pH and the concentration of a certain solution. A titration curve is drawn by plotting data attained during a titration, titrant volume on the x-axis and pH on the y-axis. The titration curve serves to profile the unknown solution. In the shape of the curve lies much chemistry and it reveals characteristic properties of the acid or base that is titrated. The titration of a strong acid with a strong base produces the following titration curve depicted in Figure 6-1. Figure 6-1 : Titration curve for the titration of a strong acid with a strong base V Base pH 7
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Acid-Base Titrations 48 Characteristic for the above curve is the sharp transition region near the equivalence point. The equivalence point for a strong acid-strong base titration curve is exactly at pH = 7 because the salt produced does not undergo any hydrolysis reactions. However, if a strong base is used to titrate a weak acid, the pH at the equivalence point will not be 7. There is a lag in reaching the equivalence point, as some of the weak acid is converted to its conjugate base. The pair of a weak acid and its conjugate base comprises a buffer , and the resultant lag that precedes the equivalence point, is called the buffering region . The titration curve for such a titration is schematically shown in Figure 6-2. Figure 6-2 : Titration curve for the titration of a weak acid with a strong base In the buffering region, it takes a large amount of base to produce a small change in the pH of the receiving solution. Also, because as its name implies the conjugate base is basic, the pH will be greater than 7 at the equivalence point. Characteristic for both titrations curves discussed above is the equivalence point , the condition in which the reactants are stoichiometric proportions. They consume each other, and neither reactant is in excess. Thus, for the number of moles n the following condition holds true (equation 6.1): added base present initially acid n n = (6.1) If the initial volume of the acid solution, as well as volume and concentration of the base added are known, the concentration of the acid can be determined (equation 6.2). V
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This note was uploaded on 04/29/2008 for the course CHEM 118 taught by Professor Jacobsen during the Spring '08 term at Tulane.

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Experiment 6 - EXPERIMENT 6 Acid-Base Titrations EXPERIMENT...

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