Experiment 20: Analysis of Vinegar
Materials:
graduated cylinder
1000 mL Florence Flask & stopper
125 mL Erlenmeyer Flasks (3)
Buret Stand (or ring stand)
Buret Clamp (or utility clamp)
Funnel, small plastic (optional)
50 mL Buret
10 mL Pipet and Pipet Bulb
100 mL Beaker
150 mL Beaker
Wash Bottle with Deionized Water
6 M NaOH: Dilute Sodium Hydroxide
KHC8H4O4 : Potassium Hydrogen Phthalate (KHP)
Phenolphthalein Indicator
Unknown Vinegar Solution (3.00-5.00 % HC2H3O2)
Preparation of Standard Sodium Hydroxide Solution
Procedure A. 1. Preparing the NaOH solution in Florence Flask
Half fill a 1000 mL Florence Flask with ~500 mL of Distilled Water.
Measure ~20 mL of 6 M NaOH into a graduated cylinder and pour into the flask.
Stopper the flask and carefully swirl to mix solution.
Procedure A. 2. Conditioning a Buret
Set up a buret.
Use small funnel and half fill the buret with NaOH solution from the Florence flask.
Allow some solution to flow out the tip; then, invert and drain the buret.
Check that there are no air bubbles in the tip of the buret.
Make
sure there
are no
bubbles,
like these,
in the tip
of the
buret.
This tip
looks good:
no bubbles!
Also, make
sure the
buret does
not leak
when
turned off.
Procedure A. 3.
Carefully fill the buret; do not overfill the buret. You’ll record an initial reading and
a final reading, then take the difference; so, it does not matter if it is at zero,
exactly; but, in this case, fill and adjust till the buret is exactly at 0.00 mL.
0.00 mL
22.90 mL
22.25 mL
0.00 mL
Procedure A. 4.
Label three 125 mL Erlenmeyer flasks #1, #2, and #3.
Accurately weigh out ~1 g of KHP into each flask.
Add ~25 mL of DI water to each flask.
Heat if necessary to dissolve the KHP crystals.
Fold the weighing paper in
half then open it to have a
crease running across.
Gently place weighing paper
on the analytical balance
Push the front bar down gently
to tare, or zero the mass.
Use a Scoopula to transfer
about 1 gram onto the
weighing paper.
Carefully remove the paper
and KHP. A crease in the
paper made this easier…
Procedure A. 5. Titration
Drain some NaOH through tip to clear bubbles.
Position Erlenmeyer flask #1 under the buret.
Record initial buret reading (± 0.05 mL)
Add a drop of phenolphthalein indicator to the flask.
titrate the KHP sample to a permanent pink endpoint
Record the final buret reading (± 0.05 mL)
Buret Stand, Buret
Clamp,Buret, and 250 mL
Erlenmeyer Flask with
sample, and 1 drop of
phenolphthalein.
Your instructor
will explain
what to watch
for during the
titration; like
the pink color
where the
drops hit the
solution, and
the persistence
of that color.
Stop the
titration when
the solution
just turns pink,
and stays pink.
Procedure A. 6. Refill the buret and repeat, record for flask #2
Procedure A. 7. Refill the buret and repeat, record for flask #3.
Procedure A. 8. Calculate the molarity of the NaOH solution for each trial; record the average
molarity of NaOH in the Data table of Procedure B.
NOTE: SAVE THE NaOH in the FLORENCE FLASK for PROCEDURE B.
Procedure B. 1.
Obtain ~50 mL of Vinegar solution in a dry 100 mL beaker. Record the unknown
number in the Data Table.
Procedure B. 2.
Condition the pipet with unknown vinegar solution, and transfer a 10.0 mL sample into
each 125 mL flask. Add ~25 mL of distilled water into each flask.
It is not necessary to use dry flasks.
Procedure B. 3.
Fill the buret with NaOH solution; adjust meniscus to zero, record initial buret reading
as 0.00 mL.
Add a drop of phenolphthalein to flask #1.
Titrate the vinegar sample to a pink endpoint.
Record the final buret reading.
Procedure B. 4.
Refill the buret to 0.00 mL, add one drop of phenolphthalein to flask #2 and titrate the
sample to a pink endpoint.
Procedure B. 5. Refill, repeat for flask #3.
NOTE: By adjusting the meniscus in the buret to 0.00 mL, the endpoints for flasks #1, #2,
and #3 should be the same.
Procedure B. 6. Calculate the molarity of acetic acid, HC2H3O2 in the vinegar solution.
Procedure B. 7. Convert the molarity of HC2H3O2 (60.06 g/mol) to mass/mass percent
concentration. Assume the density is 1.01 g/mL for the unknown vinegar solution.
NOTE: When you’re finished with this lab, please rinse the buret and all glassware with
distilled water to remove traces of NaOH solution.