DETERMINATION OF ACETIC ACID IN VINEGAR

Transcription

1 DETERMINATION OF ACETIC ACID IN VINEGAR 1 INTRODUCTION Juices from plants and fruits contain sugar. When these juices are fermented, the sugar molecules are converted into ethyl alcohol molecules (C 2 H 5 OH or C 2 H 6 O). This conversion is an example of oxidation. Fermented juices are complex solutions. The solvent is water and there are numerous solutes. The ethyl alcohol solute usually constitutes 3-12% (by mass) of the solution. Beers, wines and ciders are examples of such solutions. If oxidation of the juice is allowed to continue, ethyl alcohol will be converted to acetic acid (HC 2 H 3 O 2 or C 2 H 4 O 2 ). A fermented juice containing acetic acid instead of ethyl alcohol is called vinegar. Vinegar s primary use today is in the kitchen. The ancient Egyptians, however, used it to preserve mummies. In medieval times, it was an important medicine. To be marketed as vinegar in the United States, an aqueous solution must contain a minimum of 4% acetic acid (by mass). In today s lab, you will analyze a commercial sample of vinegar and calculate, to at least three significant figures, its actual percentage of acetic acid. This will be done by titrating your vinegar sample with the standard NaOH solution you prepared in the previous lab. PROCEDURE 1. You will need two burets for this titration: one for the vinegar solution and one for the NaOH solution. Each buret should be rinsed (a) three times with small portions of tap water, then (b) three times with small portions of deionized water, and finally (c) three times with small portions of the solution which will be put into the buret. 2. After the vinegar buret has been properly cleaned, add about 25 ml of the vinegar sample to it. Fill the properly cleaned base buret with your standardized NaOH. Record the initial volumes on the data table. 3. Dispense 5-6 ml of vinegar into a clean Erlenmeyer flask and titrate with NaOH using phenolphthalein as the indicator. Try not to overshoot the endpoint. If you do, you can back-titrate as necessary. When you have reached a satisfactory endpoint, record the final volume of each solution. 4. Repeat the titration with a second 5-6 ml sample of vinegar. If there is time, do a third titration.

2 2 DETERMINATION OF ACETIC ACID IN VINEGAR DATA Vinegar Trial 1 Trial 2 Trial 3 Final volume, ml Initial volume, ml Volume used in titration, ml Standardized NaOH Final volume, ml Initial volume, ml Volume used in titration, ml Concentration of NaOH, M (determined in previous lab)

3 3 CALCULATIONS Show your work clearly on another sheet of paper for the following calculations. 1. Determine the molarity of acetic acid in the vinegar for each titration. Then find the average molarity. 2. To find the mass percent of acetic acid in vinegar, the following equation must be used: mass of acetic acid mass of vinegar solution X 100 With a few manipulations, the molarity of vinegar can give you enough information to find both mass of acetic acid (the solute) and mass of the vinegar (the whole solution). From here on, ASSUME that you have one liter of vinegar. That way, your molarity of vinegar is very useful (molarity is moles of solute per liter of solution). With this in mind, find the mass of one liter of vinegar. The density of vinegar is g/ml. 3. Now find the mass of acetic acid in one liter of vinegar using the average molarity you found in 1. Since we are assuming one liter of vinegar, the molarity tells you how many moles are in one liter. Convert this number of moles into grams (using the molar mass). 4. You now have both components for the equation in 2: the mass of acetic acid and the mass of vinegar. Find the mass percent of acetic acid in vinegar according to your data. CALCULATIONS TABLE Molarity of HC 2 H 3 O 2 in vinegar Average molarity Mass of one liter of vinegar Mass of HC 2 H 3 O 2 in one liter of vinegar M g g Percent of HC 2 H 3 O 2 in your sample %

4 4 DETERMINATION OF ACETIC ACID IN VINEGAR Pre-lab Assignment Name Lab Section 1. In this lab you will titrate a solution of acetic acid with sodium hydroxide. Write the balanced equation for the reaction that occurs when these two compounds are combined. 2. A student performs a titration as described in the procedure. Use the following data to find the molarity of the vinegar solution. Vinegar Final volume Initial volume Standardized NaOH Final volume Initial volume Concentration ml ml ml 1.26 ml M

5 DETERMINATION OF ACETIC ACID IN VINEGAR Post-laboratory Questions 5 Name Partner 1. Calculate the percent by mass of acetic acid in a vinegar solution which is M acetic acid. Show all calculations. 2. What volume of M NaOH would be required to exactly neutralize 6.25 ml of a vinegar solution which is 3.54% acetic acid by mass? The density of vinegar is g/ml. Show all calculations.