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.
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Obtain a 25 mL transfer pipet, a 50 mL buret, 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.
Place a small quantity (about 10 mL) of NaOH solution into a clean buret.
Rotate the buret so that the solution comes into contact with the wall of the
buret. Drain the solution through the buret tip into the sink. Fill the buret with
NaOH solution to above the initial calibration line. Drain a small portion into a
beaker. Make sure the buret tip is filled with the sodium hydroxide solution.
Using a clean pipet, rinse the pipet with a small quantity of your unknown
vinegar solution. Discard the rinse solution through the tip of the pipet into
the sink. Transfer 25.00 mL of the vinegar by pipet to a clean Erlenmeyer
flask. 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.
Record the initial volume of NaOH solution in the buret to the nearest 0.01
mL.
Place the Erlenmeyer flask containing the vinegar solution under the buret.
Lower the buret so that the tip of the buret is inserted 2 cm or more into the
mouth of the flask. Place a piece of white paper under the Erlenmeyer flask.
Add 1 mL volumes of the sodium hydroxide solution from the buret 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.
Read the final buret volume to the nearest 0.01 mL.
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) Unknown number for 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.
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For the NaOH solution, 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.)
From the balanced equation, determine the number of moles of acetic
acid that must have been present in the vinegar solution.
Determine the molar concentration (molarity) of the vinegar solution.
(NOTE: You know the number of moles in 25.00 mL of solution.)
Write­Up Complete the write-up as per the format given.