Experiment: To determine the ethanoic acid (acetic acid) content of white vinegar.
Aim:White vinegar is sold as a 4% solution of ethanoic acid( acetic acid). What is the actual ethanoic acid content of white vinegar. We can determine this using an acid-base titration.
White tile/white cardboard
Indicator – phenolphthalein
Standard solution 2.0 mol/L NaOH
Titration – an experimental procedure in which the amount of some substance or ion is determined by measuring the volume of a solution of known concentration that is required to react with it.
Standard solution – a solution whose exact concentration is known accurately.
Indicator (also called an acid-base indicator) – any substance, natural or synthetic, which has one colour in acid solution and another in alkaline solution.
Pipette – used for the transfer of measured volumes of solutions for analysis, or dilution.
Burette – used to measure the volume of a titrant that reacts exactly with the sample.
Endpoint – the point at which the solution changes colour and the reaction is complete.
- Secure the burette in the clamp stand.
- Add 20mL of 2.0mol/L NaOH to the burette.
- Record the burette reading, reading from the bottom of the meniscus. This is the initial reading.
- Accurately pipette 25mL of white vinegar into the volumetric flask.
- Add 2-3 drops of phenolphthalein indicator.
- Place the white tile/cardboard on the flat surface of the clamp stand.
- Place the volumetric flask under the burette.
- Slowly add the NaOH to the volumetric flask swirling the flask constantly.
- When the colour of the solution in the volumetric flask changes to pink do not add any further NaOH. This is the endpoint of the titration.
- Record the burette reading this is the final reading.
Calculation of the results of a titration
- Write the balanced equation for the reaction, and establish the proportion in which the substances react.
- Identify the substance whose amount can be calculated from the information that is available. This information will be in the form of either the mass of a substance or the volume and concentration of a solution. Calculate the amount of this substance.
- Use the information from steps 1 and 3 to calculate the amount of the other substance.
- Complete the calculation by presenting this result in the form required, which will either be:
either the mass of the substance
or the concentration of the solution expressed in mol/L
or the concentration of the solution expressed in g/L
For the titration demonstrated:
|Concentration of standard solution of NaOH||=||2.0 mol/L|
|Titration: Final burette reading||=||31.0 mL|
|Initial burette reading||=||22.5 mL|
|Volume of NaOH solution used in titration||=||8.5 mL|
|Volume of sample (ethanoic acid) solution||=||25 mL|
CH3COOH + NaOH —–> H3COONa + H20
Amount of NaOH used in titration = 2.0 x 8.5 mol
= 0.017 mol
Since 1 mol CH3COOH = 1 mol NaOH
Amount of CH3COOH in sample = 0.0017 mol x 1000
= 0.68 mol/L
Molecular weight (MR) CH3COOH = 60
Concentration CH3COOH in vinegar = 60 x 0.68 g/L
= 40.8 g/L
Concentration CH3COOH in vinegar = 4.08g/100mL
The concentration of ethanoic acid in the white vinegar examined is consistant with the manufacturers claim of 4%.