2206 – 008

Chemistry HL PA

1

Candidate Name : Cham, Eileen Yee Lin

Candidate Number : 2206 – 008

Date of Practical : 2nd

March 2009

Practical Assessment 11 – Determination of the concentration of ethanoic acid

in vinegar

Aim

To determine the concentration of ethanoic acid, CH3COOH in vinegar (commercial vinegar, “BILAL”

brand).

Introduction

Vinegar contains a small percentage of ethanoic acid, CH3COOH. This experiment intends to find out the

concentration of the vinegar against a standard solution of sodium hydroxide solution of concentration

0.0934 mol dm-3

through acid-base titration. The equation of the reaction between ethanoic acid and

sodium hydroxide is as follows:

CH3COOH + NaOH → CH3COONa + H2O

The end point of the titration process can be determined and the amount of reactants used can be

measured. Using the values obtained from the titration, and also the chemical equation as a reference, the

concentration of ethanoic acid in the vinegar can be determined through stochiometric calculations.

Phenolphthalein indicator solution will be used in this acid-base titration.

Apparatus and materials

Commercial vinegar, “BILAL” brand

0.0934 mol dm-3

sodium hydroxide, NaOH solution

Phenolphthalein indicator solution

(50.00 ± 0.05) cm3 burette

(250.00 ± 0.12) cm3 volumetric flask with stopper

(25.00 ± 0.03) cm3 pipette

(250.00 ± 0.12) cm3 volumetric flask

250 cm3 conical flask

Retort stand with clamp

Pipette filler

2206 – 008

Chemistry HL PA

2

White tile

Wash bottle

Filter funnel

Distilled water

Procedure

1. 25 cm3 of vinegar was placed into a (250.00 ± 0.12) cm

3 volumetric flask by using a pipette and

pipette filler. Distilled water was added into volumetric flask up to the calibration mark to dilute

the vinegar.

2. The volumetric flask was stoppered and inverted several times to ensure a homogenous solution

is formed. The flask was labeled.

3. The pipette, burette, and conical flask were rinsed with distilled water.

4. The burette was then rinsed with the diluted vinegar solution, and the pipette with sodium

hydroxide solution.

5. 25 cm3 of sodium hydroxide solution was placed into the conical flask using the pipette and the

pipette filler. 1 drop of phenolphthalein indicator solution was added into the conical flask.

6. The burette was filled with diluted vinegar solution with the help of filter funnel. The initial

reading of the burette was recorded. The conical flask with its solution was placed under the

burette, and on the white tile.

7. Titration was carried out by allowing the diluted vinegar solution to flow into the conical flask

until the pink phenolphthalein indicator solution becomes colourless. The final reading of the

burette was recorded. The conical flask was swirled throughout the titration.

8. Experiment was repeated until 3 volumes of titre with difference of less than 0.1 cm3 were

obtained.

2206 – 008

Chemistry HL PA

3

Data collection – quantitative data

Volume, V

/ cm3

Rough

titration

Accurate Titration

1 2 3 4 5

Initial burette

reading, Vi

/ cm3

(± 0.05 cm3)

0.00 22.20 0.00 22.90 0.00 20.90

Final burette

reading, Vf

/ cm3

(± 0.05 cm3)

22.20 43.60 22.90 43.90 20.90 41.80

Total titre,

Vacid

/ cm3

(± 0.1 cm3)

22.2 21.4 22.9 21.0 20.9 20.9

Table 1: Collected data from titrations

Data collection – qualitative data

The originally pink solution of sodium hydroxide and phenolphthalein indicator becomes colourless when

it reaches its end point during the titration.

Data processing / analysis

Only the volumes from accurate titration 3, 4 and 5 are taken to calculate the average titre.

Average titre, Vacid = 21.0 + 20.9 +20.9

3 ± 0.1

= (20.9 ± 0.1) cm3

From the equation of the chemical reaction between diluted vinegar solution (ethanoic acid, CH3COOH

solution) and sodium hydroxide, NaOH solution,

CH3COOH + NaOH → CH3COONa + H2O

We can deduce that 1 mol of ethanoic acid is needed to neutralise 1 mol of sodium hydroxide. In other

words, the amount of moles ethanoic acid used in the reaction is equal to the amount of moles of sodium

hydroxide used.

Therefore, to find the concentration of the diluted ethanoic acid solution, Macid,

2206 – 008

Chemistry HL PA

4

Amount of moles of CH3COOH = amount of moles of NaOH

Macid × Vacid = Mbase × Vbase

Macid × 20.9 = 0.0934 × 25.00

Macid = 0.0934 × 25.00

20.9

Macid = 0.11172 mol dm-3

Where Mbase is the concentration of sodium hydroxide solution in mol dm-3

, and Vbase is the volume of

sodium hydroxide solution in cm3.

To find concentration of vinegar prior to dilution, Mvinegar,

Amount of moles CH3COOH before dilution = Amount of moles of CH3COOH after dilution

Mvinegar × V1 = Macid × V2

Mvinegar × 25.00 = 0.11172 × 250.00

Mvinegar = 0.11172 × 250.00

25.00

Mvinegar = 1.1172 mol dm-3

Where V1 is the volume of undiluted vinegar pipetted in cm3, and V2 is the volume of diluted vinegar

formed in the volumetric flask in cm3.

To find the mass of ethanoic acid in 1 dm3 of vinegar,

Mass of CH3COOH in 1 dm3 of vinegar = Mvinegar × molar mass of CH3COOH

= 1.1172 × 60.06

= 67.1 g

To find mass of ethanoic acid in 100 cm3 of vinegar,

Mass of CH3COOH in 100 cm3 of vinegar = Mass of CH3COOH in 1 dm

3 of vinegar ÷ 10

= 67.0990 ÷ 10

= 6.71 g

Therefore, the concentration of ethanoic acid in “BILAL” brand vinegar, in percentage, is 6.7099 %.

2206 – 008

Chemistry HL PA

5

Uncertainties

Percentage uncertainty in Vacid = 0.1

20.9× 100

= 0.48 %

Percentage uncertainty in Vbase = 0.03

25.00 × 100

= 0.12 %

Percentage uncertainty in V1 = 0.03

25.00 × 100

= 0.12 %

Percentage uncertainty in V2 = 0.12

250.00 × 100

= 0.048 %

Therefore,

Total percentage uncertainty = 0.48 % + 0.12 % + 0.12 % + 0.048 %

= 0.768 %

Absolute uncertainty in Mvinegar in mol dm-3

= 1.1172 × 0.768 %

= 0.009 mol dm-3

Absolute uncertainty in Mvinegar in g = 6.7099 × 0.768 %

= 0.05 g

Conclusion

The concentration of ethanoic acid, CH3COOH in “BILAL” brand vinegar, Macid is (1.117 ± 0.009) mol

dm-3

, or (6.71 ± 0.05) %. This means that for every 1 dm3 of vinegar, there is (1.117 ± 0.009) mol of

CH3COOH, or for every 100 cm3 of vinegar, there is (6.71 ± 0.05) g of CH3COOH.

The result is obtained through stoichiometric calculations between the neutralization of ethanoic acid

(which is found in vinegar) and a standard sodium hydroxide solution. Diluted vinegar solution is titrated

against sodium hydroxide solution, and the volume of diluted vinegar needed in the reaction is obtained;

then, using the obtained volume, and the chemical equation of the reaction between ethanoic acid and

sodium hydroxide as a reference, the concentration of the diluted vinegar solution can be then calculated.

The concentration of undiluted can too be derived from that.

2206 – 008

Chemistry HL PA

6

Error analysis

The literature value as provided by the teacher is 6.5 %. The result obtained from the experiment is (6.71

± 0.05) %.

Percentage error = |6.5 – 6.71

6.5| × 100

= 3.23 %

The percentage error falls outside the range of the percentage uncertainty associated with the apparatus

used in the experiment, which is 0.768 %. Therefore, the result of this experiment is not in agreement

with the literature value, and is hence not justified.

Evaluation

Some sodium hydroxide solution may have been left in the pipette when transferring into the conical

flask. This causes systematic error in the amount of moles of sodium hydroxide present in the conical

flask. The amount of moles of sodium hydroxide in the conical flask is consistently lesser than what is

expected; therefore, the amount of moles of ethanoic acid needed to neutralize the sodium hydroxide is

significantly lesser too. This affects the calculation of the final concentration of ethanoic acid solution in

the end – the value obtained should be lower than the literature value. However, we obtain a value higher

than the literature value. This suggests that there are other sources of error that contribute to the deviation

of the result from the literature value.

If the conical flask is not swirled vigorously enough throughout the titration, the solution in the flask will

cease to be homogenous and hence present a non-uniform shade – colourless in the centre of the solution,

but pink in the other parts. This shows that only the centre part of the solution is neutralized by the diluted

vinegar. It is important to swirl the flask to ensure that all parts of the solution react with the diluted

vinegar to present a uniform shade, as a non-uniform shade makes it difficult for us to detect the end point

of the titration. If the conical flask is not swirled vigorously enough, the end colour presents itself later

than it should, which results in the addition of excessive diluted vinegar. This explains the great deviation

from the average in the volumes of titre in accurate titrations 1 and 2. Also, these excessive volumes of

vinegar added explains the final concentration of vinegar obtained at the end of the stoichiometric

calculations, which is higher than the literature value.

Phenolphthalein indicator solution is colourless in both neutral and acidic solutions; therefore, we cannot

rely on the colour of the phenolphthalein indicator solution to gauge if the end point is reached. Even

though all titrations result in a clear phenolphthalein solution, which is the desired end colour, the

volumes of titre from the rough titration, accurate titration 1 and 2 are not taken into account in the

calculation. This is because although the end colours in those titrations are ideal, the acid in the solution

may be excessive. Only the volumes of titre from accurate titrations 3, 4 and 5 are used in the calculation

as they are consistent with each other (with less than 0.1 cm3 difference).

2206 – 008

Chemistry HL PA

7

Ways to improve experiment

Distilled water can be squirted down the inside of the pipette to flush down all the sodium hydroxide left

inside into the conical flask. The addition of distilled water will not affect the amount of moles of sodium

hydroxide in the conical flask, but will ensure that all sodium hydroxide is transferred into the flask.

The conical flask should be swirled vigorously throughout the experiment, and the colour change of the

phenolphthalein indicator solution should be checked closely.

The flow of diluted vinegar solution from the burette should be reduced to slow drops when the end point

is nearing. The flow should be stopped right after the drop that causes the pink phenolphthalein indicator

solution to become colourless. This is to ensure that no excessive diluted vinegar solution is added into

the conical flask.