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DETERMINATION OF ACETIC ACID IN VINEGAR
INTRODUCTION
Juices from plants and fruits contain sugar. When these juices are fermented, the sugar
molecules are converted into ethyl alcohol molecules (C2H5OH or C2H6O). 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 (HC2H3O2 or C2H4O2). 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.
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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
____________
____________
____________
Final volume, mL
____________
____________
____________
Initial volume, mL
____________
____________
____________
Volume used in titration, mL
____________
____________
____________
Standardized NaOH
Concentration of NaOH, M (determined in previous lab) ____________
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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
X 100
mass of vinegar solution
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 1.005 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 HC2H3O2 in vinegar
____________
____________
____________
Average molarity
____________ M
Mass of one liter of vinegar
____________ g
Mass of HC2H3O2 in one liter of vinegar
____________ g
Percent of HC2H3O2 in your sample
____________ %
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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
17.56 mL
12.35 mL
Standardized NaOH
Final volume
Initial volume
Concentration
20.13 mL
1.26 mL
0.1592 M
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DETERMINATION OF ACETIC ACID IN VINEGAR
Post-laboratory Questions
Name______________________________ Partner______________________________
1. Calculate the percent by mass of acetic acid in a vinegar solution which is 0.794 M acetic
acid. Show all calculations.
2. What volume of 0.0924 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 1.005 g/mL.
Show all calculations.