AP CHEMISTRY LAB
VINEGAR ANALYSIS
OBJECTIVE:
 To standardize a solution of NaOH.
 To determine the % mass of acetic acid in several different brands of vinegar.
INTRO:
The technique of titration finds many applications, but is especially useful in the analysis of acidic and basic substances.
Titration involves measuring the exact volume of a solution of known concentration that is required to react with a
measured volume of a solution of unknown concentration, or with a weighed sample of a solid. A solution of accurately
known concentration is called a standard solution. Typically, to be considered a standard solution, the concentration of
the solute in the solution must be known to 4 significant digits.
In many cases (especially with solid solutes) it is possible to prepare a standard solution by accurate weighing of the
solute, followed by precise dilution to an exact known volume in a volumetric flask. However, one of the most common
standard solutions used in acid/base titration analyses cannot be prepared in this manner. As a solid NaOH is usually of
questionable purity since it reacts with CO2 from the atmosphere and is also capable of reacting with the glass of the
container in which it is prepared in. For these reasons, NaOH solutions are generally prepared to be approximately a
given concentration. They are then standardized by titration of a weighed sample of a primary standard acidic substance
such as potassium hydrogen phthalate (KHC8H4O4), also known as KHP. KHP has one acidic hydrogen which will react
with the NaOH in a 1:1 mole ratio. From the amount of KHP used in the titration, one can determine the exact
concentration of the NaOH solution.
Commercial vinegar is supposed to be 5% acetic acid. The purpose of this lab is to analyze different brands of vinegar
and see if the manufacturers are indeed doing what they legally required to do. After standardizing the NaOH solution,
you will perform titrations of the vinegar with the standardized NaOH. The endpoint of the titration will be signaled by the
turning of phenolphthalein to pink.
MATERIALS NEEDED:
EQUIPMENT:
 3 Erlenmeyer flasks
 5 or 10 mL pipet
 pipet pump
 buret
 small beaker
 watch glass
SAFETY
PRECAUTIONS
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CHEMICALS:
 vinegar
 0.200 M NaOH
 KHP
 distilled water
 phenolphthalein
Wear safety goggles at all times while in the lab.
The KHP will be kept stored in an oven to keep moisture from adhering to the crystals.
Use tongs or a towel to remove the KHP from the oven.
NaOH is extremely caustic. Wash hands if any gets on you.
Do not pipet by mouth—use a rubber bulb or pump when pipeting.
NEEDED INFORMATION:
 The acidic compound in vinegar is acetic acid (HC2H3O2).
 The density of vinegar is 1.01 g/mL.
 The molar mass of KHP (potassium hydrogen phthalate) is 204.23 g/mol.
PROCEDURE:
STANDARDIZING NaOH:
1. Clean and dry a small beaker. Go to the oven and, using heat gloves, pour a few grams of the KHP into the beaker.
DO NOT POUR ANY EXTRA KHP BACK INTO THE BOTTLE. Take the beaker back to your lab station and allow it
to cool to room temperature.
2. While the KHP is cooling, clean 3 250-mL flask and label them 1, 2, and 3. Then pour several 5 mL samples of
NaOH into the buret and allow it to run through. Then fill the buret to the zero mark with the NaOH.
3. When the KHP is cool, weigh 3 samples of KHP between 0.6 and 0.8 g directly into your flasks. Record the masses
accordingly on the data sheet.
4. Add approximately 100 mL of H2O into each flask and dissolve the KHP completely. Then add 2-3 drops of
phenolphthalein to each flask.
5. Titrate the KHP solution in flask #1 until the endpoint is reached—the solution will turn pink. Be sure to swirl the flask
as you titrate. When the solution begins to get streaks of red, slow down the titration. If the red (or pink) persists for
more than 30 seconds, the endpoint has been reached. Record the volume of NaOH used to the nearest 0.02 mL.
6. Repeat with the other two flasks.
7. Clean all the equipment thoroughly.
ANALYZING VINEGAR:
1.
2.
3.
4.
5.
6.
Clean and dry a small beaker, and obtain 25 – 30 mL of the vinegar. Cover the vinegar with a watch glass to prevent
evaporation. Record the brand name of the vinegar on the report sheet.
Clean 3 Erlenmeyer flasks and label as sample 1, 2, and 3. Rinse the flasks with small amounts of distilled water.
Using the pump, rinse the pipet with small amounts of the vinegar and discard the rinsings.
Pipet 5 mL of the vinegar into the each of the flasks. Add approximately 100 mL of distilled water to the flasks along
with 2 – 3 drops of phenolphthalein.
Run several 5 mL samples of the NaOH through the buret. Then fill the buret with NaOH and record the initial
reading, to the nearest 0.02 mL, of the buret on the report sheet. Titrate sample 1 until the endpoint is reached.
Record the final buret reading to the nearest 0.02 mL.
Repeat the titration with the other 2 samples.
TO DO CALCULATIONS:

to determine molarity of NaOH:
o KHP has one acidic hydrogen in it. Since the net ionic equation is H + + OH− → H2O for the reaction
between NaOH and KHP, the moles of NaOH is equal to the moles of KHP.
o Molarity will equal the moles of NaOH divided by the volume of NaOH used in L.

to determine molarity of acetic acid in vinegar:
o Since acetic acid has only one acidic hydrogen, the moles of NaOH used will equal the moles of HC2H3O2
present in the solution being analyzed.
o Molarity will equal the moles of HC2H3O2 divided by the original amount, in L, put into the solution.

to determine the mass HC2H3O2 per 1 L vinegar:
o determine the mass of HC2H3O2 that would be found in 1 L of vinegar based on the molarity
 g solute = (M)(MM solute)(L solution)
o using the density of vinegar, determine the mass of 1 L of vinegar
 mass acetic acid 
 100
o mass % HC2H3O2 = 
 mass vinegar 
AP CHEMISTRY LAB
VINEGAR ANALYSIS
NAME: _______________________________________
DATA TABLE
mass KHP taken
initial NaOH buret reading
final NaOH buret reading
volume NaOH used
moles KHP present
molarity NaOH solution
average molarity and standard deviation
BRAND NAME OF VINEGAR:
SAMPLE 1
SAMPLE 2
SAMPLE 3
quantity of vinegar taken
initial NaOH buret reading
final NaOH buret reading
volume NaOH used
mole acetic acid present
molarity acetic acid in vinegar
average molarity and standard deviation
mass HC2H3O2 per 1 L vinegar
% mass acetic acid in vinegar
QUESTIONS:
1.
After checking with the other lab groups in your class, which brand of vinegar has the most acetic acid in it?
2.
Commercial vinegar is generally 5.0 ± 0.5% acetic acid by weight. Assuming this to be the true value for your
vinegar, what is your percent error?
3.
What is the formula for KHP? What is the reaction equation for it with NaOH?