Mandatory Experiment 4.3
Determination of the concentration of ethanoic acid in vinegar
Student Material
Theory
Vinegar is mainly composed of water and ethanoic acid. The concentration of ethanoic
acid in vinegar may be found by titrating a diluted solution of vinegar with standard
sodium hydroxide solution. The equation for the titration reaction is:
CH3COOH + NaOH  CH3COONa + H2O
This is a weak acid/strong base titration, and so phenolphthalein indicator is used. At the
end-point, the indicator changes colour from pink to colourless.
Chemicals and Apparatus
Vinegar
0.1 M sodium hydroxide solution
Deionised (or distilled) water
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Phenolphthalein indicator
Pipette (25 cm3)
Pipette filler
Burette (50 cm3)
Retort stand
Boss-head
Clamp
Conical flask (250 cm3)
Volumetric flask (250 cm3) and stopper
Wash bottle
White card
White tile
Beakers (250 cm3)
Dropping pipette
Filter funnel
Safety glasses
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Procedure
NB: Wear your safety glasses.
1. Place 25 cm3 of vinegar in a 250 cm3 volumetric flask and dilute with water to the
calibration mark, mixing well.
2. Stopper the flask and invert several times to ensure a homogeneous solution.
Label the flask.
3. Rinse the burette, pipette and conical flask respectively with deionised water.
4. Rinse the burette with diluted vinegar solution, and rinse the pipette with sodium
hydroxide solution.
5. Place 25 cm3 of 0.1 M sodium hydroxide solution in the conical flask, and add 3
drops of phenolphthalein indicator.
6. Fill the burette to the 0 cm3 mark with diluted vinegar.
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7. Carry out one rough and three accurate titrations.
8. Calculate the concentration of ethanoic acid in the diluted vinegar solution.
9. Calculate the percentage (w/v) of ethanoic acid in the vinegar.
Results
Rough titration result
Second titre
Third titre
Average of accurate titres
Volume of sodium hydroxide solution used in each titration
Concentration of sodium hydroxide solution
Concentration of ethanoic acid in diluted vinegar
Concentration of ethanoic acid in vinegar
Percentage (w/v) of ethanoic acid in vinegar
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=
=
=
=
=
=
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=
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Questions relating to the experiment
1. Why is the vinegar diluted?
2. Outline the correct procedure for bringing the solution in the volumetric flask
precisely to the 250cm3 mark.
3. Outline the procedure used in preparing the burette so that it is ready for the first
titration.
4. Give two other precautions which should be taken to ensure that the burette readings
are accurate.
5. Why is phenolphthalein used as the indicator in this titration?
6. Why is a rough titration carried out?
7. What happens at the end point?
8. Why are two accurate titrations carried out?
9. Calculate the percentage (w/v) of ethanoic acid in the vinegar sample.
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Teacher Material

A fuller description of titration procedure is to be found in the Student Material
relating to Mandatory Experiment 4.2.
Preparation of reagents
0.1 M sodium hydroxide solution: Use ampoules of 1 M NaOH and dilute 10 cm3 to
100 cm3 with deionised water.
Laboratory grade NaOH is not sufficiently pure to be used as a primary standard;
however it can be titrated against standard hydrochloric acid (see mandatory experiment
4.2a).
Quantities per working group
25 cm3 vinegar
150 cm3 0.1 M sodium hydroxide solution
Access to bottle of phenolphthalein solution indicator
Safety considerations

Safety glasses must be worn.
Chemical hazard notes
Solid sodium hydroxide is corrosive, and can cause severe burns to eyes and skin.
Sodium hydroxide solution irritates the eyes; wear eye protection.
When the solution is being prepared from ampoules, wear safety glasses and gloves.
Solutions of phenolphthalein may contain up to 60% ethanol and are flammable.
Disposal of wastes
Dilute with water, and flush to foul water drain.
Specimen Results
Rough titration result
Second titre
Third titre
Average of accurate titres
Volume of sodium hydroxide solution used in each titration
Concentration of sodium hydroxide solution
= 30.4 cm3
= 29.2 cm3
= 29.2 cm3
= 29.2 cm3
= 25.0 cm3
= 0.1 M
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Specimen Calculations
(a) First principles method
Volume of NaOH solution used
Moles of NaOH used
= 25.0 cm3
= 25.0 x 0.1 / 1000 moles
= 0.0025 moles
Balanced equation:
CH3COOH + NaOH  CH3COONa + H2O
1 mole
1 mole
1 mole
1 mole
Moles of CH3COOH reacting
Volume of CH3COOH solution used
Moles/ cm3 of CH3COOH used
Moles/litre of CH3COOH used
Concentration of ethanoic acid in diluted vinegar
Concentration of ethanoic acid in vinegar
Percentage (w/v) of ethanoic acid in vinegar
= 0.0025 moles
= 29.2 cm3
= 0.0025 / 29.2
= 0.0000856
= 0.0856
= 0.0856 M
= 10 x 0.0856 M
= 0.856 M
= 0.856 x 60 x 100/1000
= 5.14%
(b) Formula method
VA x MA x nB = VB x MB x nA
29.2 x MA x 1 = 25 x 0.1 x 1
MA = 25 x 0.1 x 1 / (29.2 x 1)
= 0.0856 M
Concentration of ethanoic acid in diluted vinegar
Concentration of ethanoic acid in vinegar
Percentage (w/v) of ethanoic acid in vinegar
= 0.0856 M
= 10 x 0.0856 M
= 0.856 M
= 0.856 x 60 x 100 / 1000
= 5.14%
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Solutions to student questions
1. Why is the vinegar diluted?
To avoid a very small titre, which would reduce the accuracy of the experiment.
Diluting the vinegar also reduces the amount of vinegar and the amount of sodium
hydroxide solution needed in the experiment
2. Outline the correct procedure for bringing the solution in the volumetric flask
precisely to the 250 cm3 mark.
Fill the flask to within about 1 cm of the calibration mark, and then add the water
dropwise, using a dropping pipette, until the bottom of the meniscus just rests on
the calibration mark.
3. Outline the procedure used in preparing the burette so that it is ready for the first
titration.
Rinse the burette with deionised water, and then with diluted vinegar solution. Fill
the burette with diluted vinegar solution above the zero mark. Remove the funnel.
Using the tap at the base of the burette, allow the acid to flow into a beaker until
the level of liquid is at the zero mark. Ensure that there are no air bubbles in the
nozzle of the burette.
4. Give two other precautions which should be taken to ensure that the burette
readings are accurate.
Make sure that the burette is clamped vertically. Read the level of liquid in the
burette by noting the lower level of the meniscus at eye level.
5. Why is phenolphthalein used as the indicator in this titration?
Because this is a weak acid/strong base titration, and phenolphthalein is the only
one of the commonly used indicators that changes colour in the appropriate pH
range.
6. Why is a rough titration carried out?
To find the approximate end-point. This information enables the subsequent
titrations to be carried out more quickly.
7. What happens at the end point?
The indicator changes colour from pink to colourless.
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8. Why are three accurate titrations carried out?
To reduce experimental error by calculating the average value.
9. Calculate the percentage (w/v) of ethanoic acid in the vinegar sample.
Multiply the concentration of ethanoic acid in moles per litre by 60 to get the
concentration in grams per litre. Then divide by 10 to get the percentage (w/v) of
ethanoic acid in the vinegar sample.
Extension work
Analysis of a different brand of vinegar may be done, using the same method, and the
results compared.
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