Experiment #7. Titration of Vinegar
Goal
To determine the mass percent of acetic acid in a solution via titration.
Introduction
Vinegar is a common household item that is found in a number of products from salad dressing to cleaners.
Vinegar is a solution of acetic acid (CH3COOH or HC2H3O2) in water. The amount of acetic acid is usually 5% by mass in the
vinegar solution. In this experiment, you will determine the mass percent of acetic acid in vinegar by titration.
Titration is a common method used by chemists to find the concentration of a substance in a solution. Titration
involves two key components: the titrant and the analyte. The titrant is a solution of known concentration which is used
to find the concentration of the analyte, a solution of unknown concentration. Acid-base titrations are the most common
type of titration. If the analyte is an acid, then the titrant is a base. The titrant would be added to the analyte until all of
the acid is neutralized -- this is known as the equivalence point or end-point. At the equivalence point, the number of
moles of acid (H+) is equal to the number of moles of the base (OHโ)(based on stoichiometry). By carefully measuring the
amount of titrant used, you can determine the number of moles of acid present.
The easiest way to determine the equivalence point of the reaction taking place is to use a visual indicator. Visual
indicators change color at different pH. For this titration the indicator is phenolphthalein, which changes from colorless
(pH < 8) to pink (pH > 8). The analyte for this experiment is the acetic acid in vinegar with pH less than 7. Our titrant will
be sodium hydroxide, a base. When enough titrant, NaOH, is added to neutralize the acetic acid, the solution will change
from colorless to just barely pink in color.
NaOH(aq) + HC2H3O2(aq) โ H2O(l) + NaC2H3O2(aq)
In titrations, it is important that you measure all volumes precisely. You must know the exact amount (to 0.01 mL) of
NaOH(aq) required to react with the HC2H3O2 as well as the amount of HC2H3O2 you began with.
The reaction of NaOH with HC2H3O2 is 1:1 stoichiometrically. Since you will measure the volume of NaOH(aq)
added and be given the molar concentration of the NaOH, you can find the moles of NaOH. The moles of NaOH will be
equal to the moles of HC2H3O2 in the solution. To get percent by mass of HC2H3O2 in vinegar, you will need to know the
molar mass of the HC2H3O2 and the mass of the vinegar sample.
Laboratory Activity
Materials: 2 x 125 mL Erlenmeyer flasks vinegar deionized water
50 mL buret 200 or 250 mL beaker phenolphthalein
buret stand and holder funnel standardized sodium hydroxide (NaOH) (aq)
Procedure
1. This lab will be performed individually by all students. Each student will use their own buret.
2. Rinse a 50 mL buret two or three times with deionized water. Be sure to let the water run out of the tip.
3. Remove traces of water from the buret by rinsing with NaOH(aq): add two pipettesโ worth (approximately 2 mL) of
NaOH(aq) to the buret. Drain some NaOH(aq) out of tip then pour the rest out of the top into the sink, rotating as
you pour to coat the sides with the solution. Repeat with another 2 mL portion of NaOH(aq).
