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E3 acetic acid


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  • 1. INTRODUCTION Vinegar or French for sour wine is formed by aerobic bacteria oxidizing grain alcohol to acetic acid and water. More generally, vinegar can be defined as a solution composed of acetic acid (CH3COOH), water, and, perhaps, other substances. To be sold in stores as vinegar, this solution must contain at least four grams of acetic acid per 100 ml of solution. The method used to measure the total acidity of the vinegar being studied is an analytical chemistry technique called an acid-base titration. A titration mixes two solutions which contain reactants for a known chemical reaction under conditions such that: a) the point at which both reactants have been completely consumed by the known reaction can be detected (end point) b) the amount of one reactant can be calculated from the known concentration of reactant in a standard solution, the volume of standard solution used, and the balanced known chemical equation. For an acid-base titration, the known chemical reaction in general is: Acid + Base → Water + Salt and for the titration of the vinegar in this experiment the following specific reaction will be used to calculate the acetic acid content of the vinegar sample: CH3COOH(aq) + NaOH(aq) → H2O(l) + CH3COONa(aq) Sodium hydroxide will be the standard reactant solution for this titration, and acetic acid the calculated unknown reactant. The end point in this experiment will be detected with an acid/base indicator. An acid/base indictor is a colored substance with two or more different colors depending on the value of the pH of the solution. Indicators are also very weak acids or bases and react with added acid or base if no other base or acid is present in a solution. Phenolphthalein is the indicator used in this experiment, and phenolphthalein is colorless in acid and neutral solutions but is red in basic solutions. The phenolphthalein will change color with the addition of a single drop of sodium hydroxide if no other acid (acetic acid for this experiment) is present in the phenolphthalein-sodium hydroxide solution. Another important concept in this experiment is concentration. Concentration refers to the composition of a solution and may be expressed as either the ratio of solute/solvent or solute/ solution. THEORY The method of estimation of acetic acid is based on the stoichiometric reaction with sodium hydroxide. Sodium hydroxide reacts with acetic acid to form sodium acetate and water as follow: CH3COOH(aq) + NaOH(aq) → H2O(l) + CH3COONa(aq) In the method, 1 or 2 drops phenolphthalein is used as the indicator. Appearance of first faint pink colours shows the completeness of reaction. If the strength of sodium hydroxide solution is known, the amount of acetic acid present in vinegar can be calculated from the volume required to neutralize the acid.
  • 2. TITLE Estimation Of Acetic Acid From Vinegar OBJECTIVE 1)To estimate the content of acetic acid from vinegar. 2) To prepare 0.1N of Oxalic acid solution APPARATUS Burette, Conical flask, 250ml volumetric flask, Tripod stand, Beakers Figure 1: Burette with retort stand Figure 2: Other apparatus MATERIALS Sodium hydroxide solution, Phenolphthalein, Distilled water, Vinegar PROCEDURE 1) 1.575g of Oxalic acid are weighed and dissolved in 250ml volumetric flask with distilled water to prepare 0.1N Oxalic acid solution. 2)10 ml of Oxalic acid solution is pipette out into a conical flask and 1 or 2 drops of phenolphthalein is added into the solution. The solution is titrated against alkali solution (NaOH solution) until a first faint pink color appears throughout the solution. The volume of alkali required for titration is recorded. This operation is repeated twice. 3)10 ml of vinegar is pipette out in a conical flask. 1 or 2 drops of phenolphthalein is added into it. The solution is titrated against NaOH until a faint pink color appears. The volume of alkali required for titration is recorded. This operation is repeated twice.
  • 3. RESULT Percentage Error for Burette = ±0.1 ml Table1: Titration of 0.1N Oxalic acid No. of Observation Initial Burette Reading (ml) Final Burette Reading (ml) Average Reading (ml) 1st 0.00 13.40 13.45 2nd 0.00 13.50 Table2: Titration of Vinegar solution No. of Observation Initial Burette Reading (ml) Final Burette Reading (ml) Average Reading (ml) 1st 0.00 30.80 30.80 2nd 0.00 30.80 CALCULATION  V1 = Volume needed for 0.1N Oxalic acid  S1 = Concentration of Oxalic acid  V2 = Volume of NaOH solution used in titration  S2 = Concentration of NaOH solution  V3 = Volume of vinegar used  S3 = Concentration of Acetic acid content in vinegar Concentration of NaOH used : V1S1=V2S2 (10ml)(0.1N)=(13.45ml)(S2) S2 = (10𝑚𝑙)(0.1𝑁) (13.45𝑚𝑙) = 0.0743N Therefore, the concentration of NaOH solution used is 0.0743N. Concentration of acetic acid content in vinegar : V2S2=V3S3 (30.80ml)(0.0743N)=(10ml)(S3) S2= (30.80𝑚𝑙)(0.0743𝑁) (10𝑚𝑙) = 0.229N Therefore, the concentration of Acetic acid content in vinegar is 0.229N.
  • 4. DISCUSSION In this experiment, the analyte is the Acetic Acid in the Vinegar and the titrant is a dilute solution of the strong base Sodium Hydroxide. The titration reaction is: 𝐶𝐻3 𝐶𝑂𝑂𝐻 (𝑎𝑞) + 𝑂𝐻− (𝑎𝑞) → 𝐶𝐻3 𝐶𝑂𝑂− (𝑎𝑞) + 𝐻2 𝑂 The Endpoint of the titration will be detected by observing the color change for a Phenolphthalein indicator added to the Vinegar. When the end point is reached, we can observed the colorless solution (vinegar) will change to slightly pink colour. This indicate that the solution has changed from acidic to basic. At the Equivalence Point of the Titration, we have a solution which contains predominately the Acetate Ion (CH3CO2⁻). Because Acetic Acid is a weak acid, its conjugae base, the Acetate ion, is also a weak base. This means it will partially hydrolyze in water to form OH⁻. 𝐶𝐻3 𝐶𝑂𝑂− (𝑎𝑞) + 𝐻2 𝑂 ↔ 𝐶𝐻3 𝐶𝑂𝑂𝐻 (𝑎𝑞) + 𝑂𝐻− (𝑎𝑞) In order to find the content of acetic acid from vinegar in term of concentration, hence we need to find out the concentration of Sodium Hydroxide solution by using the formula V1S1=V2S2. After the concentration of Sodium Hydroxide is known hence we can easily find out the concentration of Vinegar also by using the same formula. Apart from general sources of titration errors, when titrating acetic acid we should pay special attention to titrant. Sodium hydroxide solutions are not stable as they tend to absorb atmospheric carbon dioxide. Acetic acid is stronger than carbonic acid, so it will slowly expel carbon dioxide from the solution, but initially presence of carbonates will mean that to reach end point we need to add excess of titrant. CONCLUSION Throughout the experiment, the concentration of acetic acid from vinegar that I found is 0.229N. REFERENCES 1. Chemistry (10th Edition) by Raymond Chang 2. Chemistry: General, Organic & Biological by Janice Gorzqnski Smith 3. 4. 5.