Name________________
The Concentration of Acetic Acid in Vinegar
Introduction
Vinegar is an aqueous solution of acetic acid (a carboxylic acid). The concentration of
acetic acid in vinegar can be determined by titrating a sample of vinegar with a solution
of sodium hydroxide of known molarity. As sodium hydroxide is added to the acid an
acid – base neutralization reaction occurs when the hydrogen ion, of the acid, reacts with
the hydroxide ion, of the base, to form water. The cation of the base combines with the
anion of the acid to form a salt, in this case sodium acetate.
CH3 C = O + NaOH ----------- CH3 C = O
l
l
OH
O
Acetic Acid
Na+
+ H2O
Sodium acetate
When all of the acid is neutralized the solution will change from acidic to slightly basic
as an additional drop of sodium hydroxide is added to the flask containing the vinegar.
This change can be observed by adding an acid-base indicator, phenolphthalein, to the
vinegar prior to the titration . Phenolphthalein remains colorless when the solution is
acidic but changes to permanent pink color when neutralization occurs due to the slight
excess of base (the end point of the titration).
The molarity and percentage concentration of the acetic acid in the vinegar can be
calculated as discussed below.
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PROCEDURE
1. Using a 10.00 mL graduated cylinder, place 5.00 mL of vinegar in a clean
Erlenmeyer flask. Record the volume of the vinegar used (usually 5.00 mL).
Add 25.00 mL of distilled water to the flask using a graduated cylinder. Add 2-3
drops of phenolphthalein to the flask. (don’t forget this)
2. Set up the buret as instructed and place a white piece of paper under the
Erlenmeyer flask. Place about 100mL of NaOH of known molarity in a beaker
and carry it to your table. Rinse the buret with a small amount of NaOH. Fill the
buret with NaOH as instructed. Make sure that you allow the NaOH to run
through the buret until the volume reading is 0.00mL and there are no air
bubbles trapped in the tip of the buret.
3. Record the volume of NaOH in the buret (Usually 0.00mL). Use a white piece of
paper placed against the buret to read the bottom of the meniscus each time you
take a volume reading. Begin adding NaOH to the Erlenmeyer flask as you add
the NaOH. As you approach the end point of the titration, the pink color will
disappear more slowly. At a certain point, one drop of NaOH will cause the
entire solution to turn to a faint PERMANENT pink color. This is the end point
of the titration. Record the volume of NaOH used to reach this end point. The
volume should be recorded with two digits behind the decimal (example
12.00mL).
4. Repeat the procedure two more times using fresh 5.00mL samples of vinegar.
CALCULATIONS
The MOLARITY of the Acetic Acid can be calculated as follows:
Macid x Vacid = Mbase x Vbase
Macid = molarity of acid,
Vacid = volume of acid ,
Mbase = molarity of base
Vbase = volume of base
The molarity of the base is known (given by the instructor), the volume of the acid is
equal to the volume of the acetic acid added to the Erlenmeyer flask (5.00mL) and the
volume of the sodium hydroxide required for the neutralization can be read from the
buret. By rearranging the equation and substitution, the molarity of the acetic acid can be
determined.
The PERCENTAGE CONCENTRATION OF ACETIC ACID in the vinegar can be
calculated as follows:
The molar mass of acetic acid is 60.00 g / mole.
% acetic acid = Molarity (mol/liter) x Molar mass (g/mole)
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RESULTS and CALCULATIONS
Run 1
Run 2
Run 3
Volume of vinegar
______
______
______
Initial level of NaOH
______
______
______
Final level of NaOH
______
______
_______
_______
_______
Volume of NaOH used ______
Molarity of NaOH from instructor __________
Molarity of acetic acid ______
(from your calculation)
shown below)
________
_______
Percentage conc. of
______
Acetic acid
(from your calculations)
shown below)
_________
________
average % of acetic acid
___________________
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