Determination of concentration of ethanoic acid in vinegar by titration


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Determination of concentration ethanoic acid in vinegar by titration method. Please give proper reference to my IB student ,Eileen if you use her material.

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Determination of concentration of ethanoic acid in vinegar by titration

  1. 1. 2206 – 008 Chemistry HL PACandidate Name : Cham, Eileen Yee LinCandidate Number : 2206 – 008Date of Practical : 2nd March 2009Practical Assessment 11 – Determination of the concentration of ethanoic acid in vinegarAimTo determine the concentration of ethanoic acid, CH3COOH in vinegar (commercial vinegar, “BILAL”brand).IntroductionVinegar contains a small percentage of ethanoic acid, CH3COOH. This experiment intends to find out theconcentration of the vinegar against a standard solution of sodium hydroxide solution of concentration0.0934 mol dm-3 through acid-base titration. The equation of the reaction between ethanoic acid andsodium hydroxide is as follows: CH3COOH + NaOH → CH3COONa + H2OThe end point of the titration process can be determined and the amount of reactants used can bemeasured. Using the values obtained from the titration, and also the chemical equation as a reference, theconcentration 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 1
  2. 2. 2206 – 008 Chemistry HL PA  White tile  Wash bottle  Filter funnel  Distilled waterProcedure 1. 25 cm3 of vinegar was placed into a (250.00 ± 0.12) cm3 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. 2
  3. 3. 2206 – 008 Chemistry HL PAData collection – quantitative data Volume, V Rough Accurate Titration / cm3 titration 1 2 3 4 5Initial burette reading, Vi 0.00 22.20 0.00 22.90 0.00 20.90 / cm3 (± 0.05 cm3) Final burette reading, Vf 22.20 43.60 22.90 43.90 20.90 41.80 / cm3 (± 0.05 cm3) Total titre, Vacid 22.2 21.4 22.9 21.0 20.9 20.9 / cm3 (± 0.1 cm3) Table 1: Collected data from titrationsData collection – qualitative dataThe originally pink solution of sodium hydroxide and phenolphthalein indicator becomes colourless whenit reaches its end point during the titration.Data processing / analysisOnly the volumes from accurate titration 3, 4 and 5 are taken to calculate the average titre. 21.0 + 20.9 +20.9Average titre, Vacid = ± 0.1 3 = (20.9 ± 0.1) cm3From the equation of the chemical reaction between diluted vinegar solution (ethanoic acid, CH3COOHsolution) and sodium hydroxide, NaOH solution, CH3COOH + NaOH → CH3COONa + H2OWe can deduce that 1 mol of ethanoic acid is needed to neutralise 1 mol of sodium hydroxide. In otherwords, the amount of moles ethanoic acid used in the reaction is equal to the amount of moles of sodiumhydroxide used.Therefore, to find the concentration of the diluted ethanoic acid solution, Macid, 3
  4. 4. 2206 – 008 Chemistry HL PA Amount of moles of CH3COOH = amount of moles of NaOH Macid × Vacid = Mbase × Vbase Macid × 20.9 = 0.0934 × 25.00 0.0934 × 25.00 Macid = 20.9 Macid = 0.11172 mol dm-3Where Mbase is the concentration of sodium hydroxide solution in mol dm-3, and Vbase is the volume ofsodium 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 0.11172 × 250.00 Mvinegar = 25.00 Mvinegar = 1.1172 mol dm-3Where V1 is the volume of undiluted vinegar pipetted in cm3, and V2 is the volume of diluted vinegarformed 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 gTo find mass of ethanoic acid in 100 cm3 of vinegar,Mass of CH3COOH in 100 cm3 of vinegar = Mass of CH3COOH in 1 dm3 of vinegar ÷ 10 = 67.0990 ÷ 10 = 6.71 gTherefore, the concentration of ethanoic acid in “BILAL” brand vinegar, in percentage, is 6.7099 %. 4
  5. 5. 2206 – 008 Chemistry HL PAUncertainties 0.1Percentage uncertainty in Vacid = 20.9 × 100 = 0.48 % 0.03Percentage uncertainty in Vbase = 25.00 × 100 = 0.12 % 0.03Percentage uncertainty in V1 = 25.00 × 100 = 0.12 % 0.12Percentage uncertainty in V2 = 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-3Absolute uncertainty in Mvinegar in g = 6.7099 × 0.768 % = 0.05 gConclusionThe concentration of ethanoic acid, CH3COOH in “BILAL” brand vinegar, Macid is (1.117 ± 0.009) moldm-3, or (6.71 ± 0.05) %. This means that for every 1 dm3 of vinegar, there is (1.117 ± 0.009) mol ofCH3COOH, 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 titratedagainst 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 andsodium 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. 5
  6. 6. 2206 – 008 Chemistry HL PAError analysisThe literature value as provided by the teacher is 6.5 %. The result obtained from the experiment is (6.71± 0.05) %. 6.5 – 6.71Percentage error = | | × 100 6.5 = 3.23 %The percentage error falls outside the range of the percentage uncertainty associated with the apparatusused in the experiment, which is 0.768 %. Therefore, the result of this experiment is not in agreementwith the literature value, and is hence not justified.EvaluationSome sodium hydroxide solution may have been left in the pipette when transferring into the conicalflask. This causes systematic error in the amount of moles of sodium hydroxide present in the conicalflask. The amount of moles of sodium hydroxide in the conical flask is consistently lesser than what isexpected; therefore, the amount of moles of ethanoic acid needed to neutralize the sodium hydroxide issignificantly lesser too. This affects the calculation of the final concentration of ethanoic acid solution inthe end – the value obtained should be lower than the literature value. However, we obtain a value higherthan the literature value. This suggests that there are other sources of error that contribute to the deviationof the result from the literature value.If the conical flask is not swirled vigorously enough throughout the titration, the solution in the flask willcease 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 dilutedvinegar. It is important to swirl the flask to ensure that all parts of the solution react with the dilutedvinegar to present a uniform shade, as a non-uniform shade makes it difficult for us to detect the end pointof the titration. If the conical flask is not swirled vigorously enough, the end colour presents itself laterthan it should, which results in the addition of excessive diluted vinegar. This explains the great deviationfrom the average in the volumes of titre in accurate titrations 1 and 2. Also, these excessive volumes ofvinegar added explains the final concentration of vinegar obtained at the end of the stoichiometriccalculations, which is higher than the literature value.Phenolphthalein indicator solution is colourless in both neutral and acidic solutions; therefore, we cannotrely on the colour of the phenolphthalein indicator solution to gauge if the end point is reached. Eventhough all titrations result in a clear phenolphthalein solution, which is the desired end colour, thevolumes of titre from the rough titration, accurate titration 1 and 2 are not taken into account in thecalculation. This is because although the end colours in those titrations are ideal, the acid in the solutionmay be excessive. Only the volumes of titre from accurate titrations 3, 4 and 5 are used in the calculationas they are consistent with each other (with less than 0.1 cm3 difference). 6
  7. 7. 2206 – 008 Chemistry HL PAWays to improve experimentDistilled water can be squirted down the inside of the pipette to flush down all the sodium hydroxide leftinside into the conical flask. The addition of distilled water will not affect the amount of moles of sodiumhydroxide 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 thephenolphthalein indicator solution should be checked closely.The flow of diluted vinegar solution from the burette should be reduced to slow drops when the end pointis nearing. The flow should be stopped right after the drop that causes the pink phenolphthalein indicatorsolution to become colourless. This is to ensure that no excessive diluted vinegar solution is added intothe conical flask. 7