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olivarec - Weak Acid Titration Author Christopher Olivares...

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Weak Acid Titration Author: Christopher Olivares Martínez Instructor: Adrian de Alba Date Work Performed: February 20th, 2008 Date Report Submitted: February 29th, 2008 Abstract: The report studies the identity of an unknown weak acid through Weak Acid Titration and determines its K a value using weak acid titrations and the Henderson-Haaselbach equation. A 1.08 x10 -1 M. NaOH solution was the strong base solution to be used with the weak acid. Two tests were performed to obtain a titration plot. The moles of weak acid were 2.91 x10 -3 mol. The acid was determined to be Trans-Fumaric Acid. The K a was determined to be 4.58 and its calculated molecular weight was 83.79 g/mol. 1
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I. INTRODUCTION _______________________ ___________________________________ The objective of this experiment is to determine the identity and dissociation equilibrium constant (K) of an unknown acid by performing weak acid titrations. The volume of base being added to the acid and the pH will be recorded. Some of the acids that will be available are diprotic, therefore two pK values are expected in those cases. Acid Base equilibrium is a recurring concept in chemistry. Acid and bases both dissociate in water. However, the extent to which they dissociate depends on the stability of the ions they form. Strong bases and strong acids have a high tendency to ionize. Acids and bases also react forming salts and water as seen in eqn. (1-1). This process is called neutralization. In this experiment, an unknown weak acid will react with a strong base, NaOH. It is expected that the reaction will proceed to completion and NaOH will neutralize the acid. The last endpoint the moles of the unknown acid will be equal to the moles of base and the pH will have a close value to the neutral point (pH 7). O H BA OH B HA 2 ) ( + → + The Henderson-Haaselbach equation (eqn 1-2) will be used to determine the pK a of the weak acid. (A - ) is the conjugate base to the weak acid (HA). + = - ] [ ] [ log HA A pK pH a At half the equivalence point the reaction will be at half-life. Thus, [A - ] and [HA] will have the same value and the pH = pK a . Since a strong base will be reacting with a weak base, the strong base (NaOH) will be a reagent in excess. NaOH dissociates well in water and will make the pH basic. In other words, when the titration plot is over, the solution is expected to contain salt(s), water, Na + , and OH - .Obtaining the K a and the equivalence points from the titration plot, the acid could be identified by comparing it to literature values. K a will be verified by comparing it to the pH at half-life (when pH = pK a ).
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