Titration of Vinegar s1

Titration Of Vinegar Introduction

Quantitative analysis determines the amount of a particular substance in a sample. This determination is accomplished through a titration of the unknown sample with a solution of known concentration. The substances must react.

An indicator is a chemical which does not take part in the reaction but signals the completion of the reaction. The indicator chosen will have one color before the reaction is complete and another color when completion occurs.

A balanced equation and stoichiometric analysis will determine the concentration of the unknown solution. Theory The acetic acid content of a vinegar may be determined by titrating a vinegar sample with a solution of sodium hydroxide of known molar concentration (molarity).

CH3COOH(aq) + NaOH(aq) --> CH3COONa(aq) + H2O(l) (acid) + (base) --> (salt) + (water)

Phenolphthalein will be used as an indicator because it will be colorless before the completion of this reaction but pink after the completion. Phenolphthalein, an organic dye, is colorless in an acid solution and pink in a basic solution. You must be prepared to search carefully for a point in the titration at which one drop of the NaOH solution will cause the solution being titrated to turn from colorless to a barely discernible pink color. This point is called the end point and indicates the reaction is complete. Procedure

1. Obtain a 10 mL graduated cylinder, a 50 mL burette, a known NaOH solution, and an unknown vinegar solution. The NaOH solution must be kept in a stoppered bottle, otherwise, it will react with CO2 in the air. 2. Priming your Burette (Rinse#1) Add about 10 mL of distilled water. Rotate the burette so that the water comes into contact with the wall of the burette. Drain the water through the burette tip into the sink. 3. (Rinse #2)Place a small quantity (about 10 mL) of NaOH solution into a clean burette. Rotate the burette so that the solution comes into contact with the wall of the burette. Drain the solution through the burette tip into the sink. (Rinse #3) Repeat step 3 on more time. 4. Fill the burette with NaOH solution to above the initial calibration line. Drain a small portion into a beaker. Make sure the burette tip is filled with the sodium hydroxide solution. 5. (Setting up the Unknown)Record the mass of the empty Erlenmeyer flask. Transfer 10.00 mL of the vinegar using a graduated cylinder to a clean Erlenmeyer flask. Record the mass of the Erlenmeyer flask and solution. Add 40 mL of distilled water from a clean graduated cylinder to the vinegar solution. Add three drops of phenolphthalein indicator solution to the vinegar solution. 6. Record the initial volume of NaOH solution in the burette to the nearest 0.01 mL. 7. Place the Erlenmeyer flask containing the vinegar solution under the burette. Lower the burette so that the tip of the burette is inserted 2 cm or more into the mouth of the flask. Place a piece of white paper under the Erlenmeyer flask. 8. Add 1 mL volumes of the sodium hydroxide solution from the burette to the vinegar sample while swirling the flask. As the titration proceeds, a pink color will be observed at the point where the sodium hydroxide solution contacts the vinegar solution. Near the end point of the titration, the pink color flashes through the solution and the color will remain for increasingly longer periods of time. When this is observed, add the sodium hydroxide solution drop by drop. The titration is complete when the pink color persists throughout the solution for 30 seconds. 9. Read the final burette volume to the nearest 0.01 mL. 10. Repeat the above procedure twice for a total of 3 titrations. The volumes of NaOH solution used in at least two titrations should not differ by more than 0.15 mL. If there is a discrepancy, repeat the titration. Data Be sure to record: a) Mass of the vinegar solution. b) The volumes of vinegar solution. c) The initial and final volumes of the NaOH solution from the buret to the nearest 0.01 mL for each titration. This should be in table form. Calculations 1. Find the number of moles of base, use the molarity (molar concentration) given and the volume of NaOH used to determine the number of moles of NaOH used in the titration. (Average the volumes of NaOH solution.) 2. From the balanced equation, determine the number of moles of acetic acid that must have been present in the vinegar solution.(dimensioinal Analysis) Theoretical yield. 3. Determine the molar concentration (molarity) of the vinegar solution. (NOTE: You know the number of moles in 10.00 mL of solution.) 4. Determine the %acid in the vinegar solution (from yesterday’s notes).