Vinegar Analysis Formal Report

Vinegar Analysis Formal Report Introduction The common problem in chemistry is to determine the concentration of an unknown solution. The purpose of vinegar analysis lab experience is to determine the acetic acid CH3COOH concentration and the mass percent of acetic acid in vinegar. A titration is a common analysis method that determines the concentration of an unknown solution with one known concentration solution. In the vinegar analysis lab experience, sodium hydroxide NaOH with known concentration is used to titrate acetic acid solution (vinegar). In a neutralization reaction, H+ from the acetic acid combine with OH- from sodium hydroxide produce water. In this chemical reaction, the amount of sodium hydroxide is completely neutralized with acetic acid called equivalent point. H+(aq) + OH- (aq) H2O(l) The pH indicator, phenolphthalein is a clear colorless in acid solution but shows pink color in basic solution.1 When the equivalent point is reached, add one more drop of sodium hydroxide the phenolphthalein will change color to pink which reaches the end point. Though the end point happens right after the equivalent point is reached, an extra drop of titration is required for calculation purpose.2 Experimental To start the titration lab experiment, first of all, add 100 mL of 0.02985 M NaOH (the NaOH that is mentioned below has the same molarity) to a 250 mL beaker. Rinse the inner wall of the buret with NaOH three times (or for three minutes), allow the NaOH solution to drain through the stopcock. After rinsing the buret, fill the buret with the NaOH solution to the zero mark roughly 1mL above. Next, measure 10 mL of vinegar with a graduated cylinder then add it to a clean 250 mL beaker. Add 2 or 3 drops of phenolphthalein to the vinegar. Place the beaker with vinegar under the buret. It could be potentially easier to notice the color changes if placing the beaker on top of a piece of white paper when the end point is reached. Slowly add 1 mL of the NaOH solution at a time until the end point is reached at which time the solution turns pink. Weigh each of the three 125 or 250 mL flasks. Then add 10 mL vinegar to each of the three flasks and weigh the flasks after adding vinegar. Add 10 mL of ID water to each of the flasks. Repeat above titration process three times. Data and Calculation The data is collected from the above procedure is listed in Table 1 and 2. In Table 1, the moles of acetic acid are equal to the moles of NaOH which are obtained from NaOH volumes measurements according to Eq (1). To acquire the mass of acetic acid in vinegar, calculate with moles of acetic acid according to Eq (2). Using Eq (3) to get the mass percent of acetic acid with the mass of acetic acid and mass of vinegar. The molarity of NaOH was given in the lab. CH3COOH(aq) + NaOH(aq) CH3COONa(aq) + H2O(l) Moles CH3COOH = Moles of NaOH =Molarity of NaOH x Volume (L) of NaOH (1) Mass of CH3COOH = Moles of CH3COOH x Mole Mass of CH3COOH (2) Mass % of acetic acid in vinegar = (Mass of acetic acid / Mass of vinegar) x 100% (3) Table 1: Standardizationn of NaOH Solution Trial 1 Trial 2 Trial 3 Molarity of NaOH 0.2985M +- 0.002 0.2985M +- 0.002 0.2985M +- 0.002 (mol/L) Mass of vinegar (g) 10.13 9.91 10.11 Volume of NaOH 28.78 28.40 28.99 delivered (ml) Moles of NaOH and 8.591x10-3 8.474x10-3 8.654x10-3 acetic acid (mol) Mass of acetic acid in 0.5158 0.5088 0.5190 5.134 5.134 vinegar (g) Mass % of acetic acid 5.092 in vinegar (%) Table 2 summarizes results from Table 1: mean and standard deviation for acetic acid in vinegar. The present error of the lab experiment is 0.024%. Table 2: Statistical Analysis of Acetic Acid in Vinegar Acetic Acid in Vinegar Mean mass % 5.120 Standard deviation of mass % 0.03834 Manufacturer listed mass % 5.00 % error of mass % 0.024 Results and Discussion In this experiment, a sample of vinegar was titrated with 0.2985M±0.002 NaOH solution. The experiment was repeated three trials to achieve a more accurate result and less percentage error. The results were calculated the mean mass percent of acetic acid in the vinegar sample was 5.12±0.04 %. This gave 0.12±0.04% difference compared to the 5% manufacturer listed mass percent of acetic acid in vinegar. One possible source of error in this experiment was caused by additional NaOH solution in the titration. It was difficult to reach the exact endpoint; often the last drop of NaOH added causes the color change from clear to dark pink rather than pale pink color. The additional drop of NaOH solution increases the moles of NaOH used in titration, resulting in an incorrectly high mass percent of acetic acid in vinegar. Another possible source of error was the loss of NaOH solution: some of the NaOH solutions could remain on the wall of the flask, and occasionally scant amount of NaOH remains adherent in buret’s tip at the end of titration. The excess of NaOH results as over-titration. To avoid over-titration, the flasks were rinsed with DI water before procedures started, and the buret was turned off quickly as the endpoint was reached. Another source of error may also occur with the loss of some vinegar when transferring the solution. The lower volume of vinegar would require less NaOH for titration. The incorrect NaOH volume results in a lower mole of NaOH which causes an incorrect low mass percent of acetic acid. Conclusion In this experiment, the vinegar sample was titrated with a standard 0.2985 ±0.002 NaOH solution. In conclusion, based on the data collection, the average mass percent concentration of acetic acid was 5.120%, which is 0.12% difference compared to the manufacture listed mass percent of acetic acid in the vinegar sample. The standard deviation is 0.03834 which is fairly low. The result of the experiment is accurate because the percent error of the mass percent of acetic acid in vinegar is 0.024% less than 5%. This experiment is significant because being able to determine the concentration of an unknown solution with one known concentration solution is necessary and important in scientific experiments