Chem

Irene Wu

Laboratory 5. Acetic Acid Concentration in Vinegar Report

        The goal of this labortory is to understand volumetric analysis and the quantitative measurement of solution concentrations. The titration of acetic acid in vinegar is performed using a sodium hydroxide solution which has been standarized.

        The experiments performed are preperation and standarization of sodium hydroxide solution and determination of acetic acid content of vinegar. First, 2.5-2.7 grams of sodium hydroxide was weighed on a top-loading balance and transfer into a 250 mL erlenmeyer flask. The flask was rinsed with distilled water before. Distilled water is added to the flask that the liquid level in the flask is approximately 150 mL. The flask was swirled until the sodium hydroxide is fully dissolved. The solution is diluted after the sodium hydroxide pellets have dissolved to a volume of 200 mL. The flask is swirled by hand to be well mixed. Next, 1.0 grams of potassium hydrogen phthalate is weighed on an analytical balance. The KHP is transfer to a 250 mL erlenmeyer flask with a magnetic stir bar. 50 mL of distilled water is then added to the flask and 2-3 drops of phenolphthalein is added to the flask. Any KHP that was on the side of the flask was washed with a small amount of distilled water. 5-10 mL of sodium hydroxide solution is used to wash the buret with three times. The buret is placed into the buret clamp and filled just below the highest volumetric mark. The magnetic stir bar in the flask is started with the buret tip inside the neck of the erlenmeyer flask. The stopcock is opened and allowed the sodium hydroxide solution into the flask initially transferring 5 mL of solution at a time. The color is observed and the rate is slowed down as the color of the indicator takes longer to fade. When it was close to the endpoint, the sodium hydroxide solution is added drop by drop. Only a minimum amount of sodium hydroxide is wanted to change the solution to pink. The KHP standardization of the sodium hydroxide solution is repeated two more times and the number of moles of KHP used is calculated and from that the sodium hydroxide solution concentration for each trial and the average value of the three trials is determined. When determining the acetic acid content of vinegar, 10.00 mL of vinegar is pipette into a 250 mL erlenmeyer flask. The vinegar is combined with 50 mL of distilled water and 2-3 drops of phenolphthalein indicator solution. The stopcock is open to titrate the vinegar solution in the same manner used to titrate the KHP in the standardization procedure. It only takes 1 or 2 drops to change the color from colorless to light pink. The procedure is repeated 2 more times to provide three values for the acid concentration of the vinegar samples. The concentration of acetic acid in vinegar is calculated and averaged of the three measurements.

        Titration is a useful method that the materials used react completely and quickly. Titration is used for acid-base reactions where hydroxide ions and hydronium ions combine to form water in a neutralization reaction. It involves to measure the volume accurately of a solution required to neutralize the acid or base being titrated. A buret is used to accurately measure the amount of acid or base used to neutralize the analyte. The phenolphthalein is an indicator to change the color to pink. It is colorless in acidic solution and pink in basic solution. The results of the mass of sodium hydroxide used is 2.568 g. The final volume of sodium hydroxide solution is 200 mL. The masses of KHP used in three trials are 1.0433 g, 1.0634 g, and 1.0088 g. The moles of KHP are 0.014 mol, 0.015 mol, and 0.014 mol. The initial volume for the three trials is 50.00 mL. The final volume for trial 1 is 33.00 mL, trial 2 is 34.80 mL, and trial 3 is 35.40 mL. The volume of vinegar used in the acetic acid content of vinegar is 10.00 mL in all 3 trials. The initial volume of distilled water is 50.00 mL. The final volume of trial 1 is 18.50 mL, trial 2 is 12.00 mL, and trial 3 is 22.50 mL. The concentration of an unknown solution can be calculated if the volumes of the standard and unknown solutions used in the titration and the concentration of the standard solution are known. It is hard to obtain solid sodium hydoxide in pure form because it absorbs water from the air and its solution reacts with carbon dioxide. Sodium hydroxide must be standarized before it can be used to be accurate. It can be standardized by titrating it against an acid solution of accurately known concentration. The acid used for this laboratory is potassium hydrogen phthalate and it is a soluble solute in pure form.  A titration of a known, measured amount of potassium hydrogen phthalate with the unknown concentration of sodium hydroxide will give a volume of sodium hydroxide solution required to neutralize the potassium hydrogen phthalate. The total number of moles of H+ ions that have been reacted at the equivalence point must be equal to the total number of moles of OH- ions that have reacted.