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Ateneo de Manila University School of Science and Engineering Department of Chemistry
TITRATION OF COMMERCIAL VINEGAR INTRODUCTION
Vinegar is prepared by fermenting sweet fruit juices (e.g. pineapple, grape, apple, sugar cane) and sap from coconut and nipa. The active component in vinegar is acetic acid, CH3COOH or HAc (where Ac = CH3COO-) Government standards specify a minimum acetic acid concentration of 4.5 % by weight or 0.75 M. To determine the acetic acid content of vinegar, it is titrated with a base of known concentration, such as $sodium hydroxide. The acid-base reaction is expressed as follows: HAc acetic acid
NaOH sodium hydroxide
NaAc
HOH
sodium acetate
water
The equation shows that acetic acid and sodium hydroxide react in equimolar (1 mole acid is to 1 mole base). If we use sodium hydroxide of known concentration, we can determine the concentration of acetic acid because we can readily determine the number of moles of sodium hydroxide needed to neutralize the acid in a given amount of vinegar sample. MATERIALS
sample of commercial vinegar-to be brought by the student 0.5 M NaOH solution phenolphthalein indicator 50-mL Erlenmeyer flask 10-mL graduated cylinder dropper bottle PROCEDURE
Part I. Calibration of dropper Bottle 1. Fill a dropper bottle with water. 2. Fill a 10-mL graduated cylinder with water up to the 9-mL mark. 3. Bring water up to the 10-mL mark by careful addition of water from the dropper bottle. Record the number of drops used. 4. Perform three trials and determine the number of drops equivalent to 1 mL. Part II. Titration of Vinegar 1
1. Weigh a 50-mL Erlenmeyer flask. 2. Measure out 3 mL of commercial vinegar sample into the pre-weighed flask. 3. Weigh the flask and vinegar. Record the weight. Then add 1 drop of phenolphthalein. 4. Triple wash the calibrated dropper bottler with a small amount of 0.5 M NaOH. Fill the dropper bottle with the same base solution. 5. Titrate the vinegar sample with the base. Note the number of drops (volume) of base used. 6. Perform 3 trials. Use the results of the three trials to obtain the average concentration of acetic acid in the vinegar sample. CALCULATIONS
1. Calculate the concentration (molarity) of acetic acid in your vinegar sample. (Macid)(Vacid) = (Mbase)(Vbase) 2. Calculate the percentage of acetic acid in 3 mL vinegar sample. % HAc = {(Mbase)(Vbase)} x MW of acetic acid x 100 weight vinegar sample
QUESTIONS 1. Why did you need to calibrate your dropper bottle? 2. What do you observe when you do not swirl the solution after adding several drops of NaOH? 3. From your results, does the vinegar conform to government standards? Compare your results with others who analyzed the same sample. What conclusions can you make out of this? 4. What are the possible sources of error in this experiment? 5. Would the reported molarity of the vinegar sample be affected by an error in weighing? How about the % HAc? 6. Compare different brands of vinegar used in class. Which has the highest concentration of acetic acid? the lowest?
REFERENCE Abenojar, E.C., et al. (2007). General Chemistry 2 Laboratory Manual. Chemistry Department, Ateneo de Manila University. 23.
