4.3 - Determination of the Concentration of Ethanoic Acid in Vinegar

infoNote

Experiment Summary

This experiment aims to determine the concentration of ethanoic acid ( $CH₃COOH$ ) in vinegar by titrating it with a standard solution of sodium hydroxide ( $NaOH$ ).

The reaction between ethanoic acid and sodium hydroxide is a weak acid-strong base reaction, producing sodium acetate and water.

Phenolphthalein is used as the indicator, changing from pink to colourless at the endpoint.
The titration results are used to calculate the concentration of ethanoic acid in the vinegar and its percentage by weight/volume (w/v).

Materials and Apparatus Required

Chemicals

Vinegar
0.1 M sodium hydroxide solution
Deionised (or distilled) water
Phenolphthalein indicator

Apparatus

Pipette (25 cm³) and pipette filler
Burette (50 cm³) with retort stand and clamp
Conical flask (250 cm³)
Volumetric flask (250 cm³)
Wash bottle
White card or tile
Beakers (250 cm³)
Dropping pipette
Filter funnel
Safety glasses

Safety Precautions

Wear safety glasses throughout the experiment.
Sodium hydroxide is corrosive and can cause burns; handle with care and avoid contact with skin.
Phenolphthalein contains ethanol and is flammable; keep it away from open flames.
Dispose of waste by diluting it with water and flushing it down the drain.

Method

Place 25 cm³ of vinegar into a 250 cm³ volumetric flask.
Dilute with deionised water to the calibration mark, then stopper and invert the flask several times to ensure a homogeneous solution.
Rinse the burette, pipette, and conical flask with deionised water.
Rinse the burette with diluted vinegar and the pipette with sodium hydroxide solution.
Use the pipette to transfer 25 cm³ of 0.1 M sodium hydroxide solution into the conical flask.
Add 3 drops of phenolphthalein indicator.
Fill the burette with a diluted vinegar solution, ensuring the level is at the 0 cm³ mark.
Perform a rough titration by adding vinegar from the burette while swirling the conical flask until the solution changes from pink to colourless.
Record the volume of vinegar used.
Perform three accurate titrations by quickly adding vinegar until near the endpoint, then adding it dropwise until the colour change occurs.
Record the volume of vinegar used for each accurate titration.
Calculate the concentration of ethanoic acid in the diluted vinegar solution and then the concentration in the original vinegar sample.
Calculate the percentage (w/v) of ethanoic acid in the vinegar.

Results

Measurement	Value
Rough titration result	30.4 cm³
Second titre	29.2 cm³
Third titre	29.2 cm³
Average of accurate titres	29.2 cm³
Volume of sodium hydroxide solution	25.0 cm³
Concentration of sodium hydroxide	0.1 M
Concentration of ethanoic acid in vinegar	0.856 M
Percentage (w/v) of ethanoic acid	5.14%

infoNote

Sample Calculation

Using the formula:

\text{Moles of NaOH} = \frac{25.0 \times 0.1}{1000} = \highlight[0.0025 \, \text{moles}]

From the balanced equation:

\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}

Moles of ethanoic acid = 0.0025 moles.

The concentration of ethanoic acid in diluted vinegar:

\frac{0.0025}{29.2/1000} = \highlight[0.0856 \, \text{M}]

The concentration of ethanoic acid in original vinegar:

0.0856 \times 10 = \highlight[0.856 \, \text{M}]

Percentage (w/v) of ethanoic acid in vinegar:

0.856 \times 60 = \highlight[5.14\%]

Example Questions with Answers

infoNote

Q1: Why is the vinegar diluted before the titration?

To avoid a very small titre, which could reduce accuracy.

Diluting also reduces the amount of vinegar and sodium hydroxide needed for the titration.

infoNote

Q2: How do you bring the solution in the volumetric flask to exactly 250 cm³?

Fill the flask to within 1 cm of the calibration mark, then add deionised water dropwise using a pipette until the meniscus rests on the mark.

infoNote

Q3: Why is phenolphthalein used as the indicator in this titration?

Phenolphthalein is suitable for weak acid-strong base titrations, changing colour in the appropriate pH range.

infoNote

Q4: Why are three accurate titrations carried out?

To reduce experimental error and obtain an average value.

infoNote

Q5: What happens at the endpoint?

The phenolphthalein indicator changes colour from pink to colourless, indicating neutralisation has occurred.

- Determination of the Concentration of Ethanoic Acid in Vinegar Simplified Revision Notes for Leaving Cert Chemistry

4.3 - Determination of the Concentration of Ethanoic Acid in Vinegar

Experiment Summary

Materials and Apparatus Required

Chemicals

Apparatus

Safety Precautions

Method

Results

Sample Calculation

Example Questions with Answers

Q1: Why is the vinegar diluted before the titration?

Q2: How do you bring the solution in the volumetric flask to exactly 250 cm³?

Q3: Why is phenolphthalein used as the indicator in this titration?

Q4: Why are three accurate titrations carried out?

Q5: What happens at the endpoint?

500K+ Students Use These Powerful Tools to Master - Determination of the Concentration of Ethanoic Acid in Vinegar For their Leaving Cert Exams.

Other Revision Notes related to - Determination of the Concentration of Ethanoic Acid in Vinegar you should explore