Is Household Vinegar Really 5%?
The quantity of acid in a sample of vinegar may be found by titrating the sample against a standard basic
solution. Titration is the process for ascertaining the exact volume of a solution that reacts according to a
balanced chemical equation with a given volume of known concentration of a second solution. The endpoint
occurs when stoichiometric quantities of the reagents have been mixed. The endpoint of a titration for reactions
of acids and bases is usually indicated by a third reagent, the indicator, which has an abrupt and distinctive color
change at the hydrogen ion concentration which is present after neutralization has occurred. A good indicator
of choice is phenolphthalein. Phenolphthalein is colorless in acidi~ solutions and red (pink in dilute solutions)
in basic solution. Since it is much easier and distinctive to see a color change from colorless to pink rather than
from red to pink to colorless, the sodium hydroxide will be added' to household vinegar which already contains
the phenolphthalein. Most commercial preparations of vinegar have a mass percentage of between 4.0% and
5.5% acetic acid. By determining the volume of sodium hydroxide solution of known concentration necessary
to neutralize a measured volume of vinegar, the concentration of the vinegar can then be calculated.
MATERIALS
distilled water
5 mL commercial vinegar .
1% phenolphthalein indicator
three 1 mL micro-tip pipets
toothpick
5 mL 1.0 M NaOH
one 24-welI plate
two 10 mL graduated cylinders
PROCEDURE: WEAR EYE PROTECTION AT ALL TIMES. SODIUM HYDROXIDE IS CORROSIVE TO
THE FLESH AND CAN CAUSE BLINDNESS. Wash all spills immediately using large amounts of water. Know
where the eye wash station is located.
Part One: Calibrating Pipets
1.
Clean and thoroughly dry the 10 mL graduated cylinders.
2.
Label one pipet 1 Molar NaOH. Label another pipet vinegar and fill each.
3.
Determine the number .of milliliters per drop for each of the pipets. Do this by holding the pipet "in a
vertical position and let each drop fall into the graduated cylinder by gently squeezing the bulb.
Complete the table below before you go further. Compute averages to 3 significant figures. You will
later round your final answer to 2 significant figures.
4.
Use the appropriate solutions in the graduated cylinders to refill the pipets for part two.
DATA TABLE
Vinegar
50 drops
100 drops
150 drops
200 drops
50 drops
100 drops
150 drops
200 drops
Volume in mL
Average mLjdrop
Sodium
Hydroxide
Volume in mL
Average mLjdrop
Jsmg the data from the last column compneted record the average ( op SIZeot the vinegar and sodium nvdroxide
y
solution below.
Vinegar drop size (mL)
NaOH drop size (mL) :--:,--.,.-:-_
To obtain how many drops of vinegar are equivalent to a drop of sodium hydroxide solution divide the size of
a drop NaOH by the size of a drop of vinegar. Round to 3 significant figures. This is your correction factor.
drops of vinegar = 1 drop of sodium hydroxide solution
----
Part Two: Determining the Molarity of your Vinegar
1.
2.
3.
4.
5.
6.
Use five wells of a 24-well plate adding 25 drops of vinegar to each well. Make sure that all the drops
fall directly to the bottom of the wells. Add 2 drops of phenolphthalein indicator to each of the five
wells, Place the 24-well plate on white paper.
Stir the vinegar and phenolphthalein mixture with the toothpick.
Select one of the wells. Counting the drops, add sodium hydroxide solution to this well, dropwise with
stirring, until a faint pink color is observed that remains for at least 30 seconds. Be sure to hold the
pipet in the same manner for each drop to help assure that the drop size is uniform. Record the
number of drops required on the data table below.
Repeat this procedure [steps 1-4] for the other 4 portions of vinegar.
Check your data. All five titration pairs should agree within 1 drop of one another. If they do not,
repeat the titration.
Clean all your equipment and wash any left over acid or base down the sink.
DATA TABLE
Trial #
Drops of
vinegar
Drops of
indicator
Drops of
NaOH
Drops of
NaOH,
adjusted for
drop size
concentration of
vinegar
wt. % of
vinegar
one
two
three
four
five
DATA ANALYSIS
From the coefficients in the balanced equation for the reaction between acetic acid and sodium hydroxide,
complete the following mathematical relationship:
____
moles of acetic acid
'=
moles of sodium hydroxide
Adjust your drops of NaOH by multiplying the actual drops by the correction factor you determined earlier. Use
the following equation with your experimental data to determine the concentrations of acetic acid in each of the
trials.'
[CH COOH] '= [NaOH] x adjuste~ drops of NaOH
3
drops of VInegar
Average the values of the concentration of acetic acid in vinegar and round to 2 significant figures. Are any of
the experimental concentrations very different from the average? If so, suggest some possibilities of experimental
error to explain their dissimilarity. Compute the average without including widely discrepant data.
Assuming the density of vinegar to be about the saine as that of pure water, determine the weight percent of
acetic acid in vinegar from the concentration determined above. Use the unrounded figures in your calculations
and then round to two places. Compare that value to the weight percent listed on the bottle.
LAB QUESTIONS:
1.
Write the balanced net ionic equation for the neutralization of acetic acid with sodium hydroxide.
2.
A 1 mL sample of dilute acetic acid solution required 4 mL of .2 M NaOH solution for complete
neutralization. The density of the H~HP2 solution was 1.05g/mL. Calculate the % of HC2H302 in
the solution.
.
3.
If you use the same vinegar in each well and if each well is titrated with the same sodium hydroxide
solution, why is it necessary to titrate five samples of the vinegar?