Exp4 - Acid,Base Titration

Exp4 - Acid,Base Titration - ACID/BASE TITRATION EXPERIMENT...

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1 ACID/BASE TITRATION EXPERIMENT 4 INTRODUCTION In PART I of this experiment, you will determine the concentration (molarity) of a sodium hydroxide solution (NaOH) by titrating an acid, the potassium biphthalate (HOOCC 6 H 4 COOK). The equivalence point will be determined by the change in color of the Phenolphthalein indicator. Knowing that one mole of NaOH neutralizes one mole of HOOCC 6 H 4 COOK, the molarity of the sodium hydroxide solution can be determined from the volume of base added to reach the equivalence point, and the weigh of biphthalate acid used. The accurate determination of the NaOH solution molarity is called standardization . After PART I, the NaOH solution will be called Standard NaOH solution. In PART II of this experiment, you will determine the molarity of an unknown weak acid by a pH- titration with your own Standard NaOH solution. Measuring the pH of the solution and the volume of NaOH added as the titration proceeds, a titration curve (graph) will be constructed. The equivalence point, the pK a and K a of the unknown acid can then be determined from the curve. Mastering the techniques when using the pipet, buret and analytical balance will be crucial here. THEORY Acid/base titration is the process of mixing measured volumes of acid and base solutions in such a manner that you can determine when , equivalent amounts (moles) of each are present. The purpose of titration is to determine the concentration of one solution (the base in PART I of this experiment), while the concentration of the other (the acid) is known to a high degree of accuracy. The reaction of an acid HA with a base B is called neutralization , and is represented by equation (1). HA + B A + BH + (1) An acid/base neutralization reaction is simply a proton ( H + ) transfer. The acid acts as a proton donor , the base as a proton acceptor . The equivalence point of a titration is the point at which equivalent amounts (number of moles) of acid and base have been mixed. In order to determine the equivalence point , a visual indicator is added to the solution to be titrated. When properly selected, the indicator undergoes a sharp color change slightly after the equivalence point . So slightly in fact that we consider the color change to happen precisely at the equivalence point.
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2 In PART I of this experiment, a precisely known (weighed) amount of acid HA is placed into an erlenmeyer flask, along with a few drops of indicator HInd , water, and a magnetic stir bar. The flask is then set under a buret, as shown in Figure 1. The buret is filled with base B solution. While the base is slowly added to the erlenmeyer flask, reaction (1) occurs as long as HA is present in the flask. When the last molecules of acid HA have reacted, the next drop of base B will react with the indicator as shown by reaction (2), and the solution will turn pink. HInd +
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This note was uploaded on 04/04/2011 for the course SCIENCE CHEM 120 taught by Professor Fenster during the Winter '11 term at McGill.

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Exp4 - Acid,Base Titration - ACID/BASE TITRATION EXPERIMENT...

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