Experiment Number 3 CHM12-3L Stoichiometry of Solutions

EXPERIMENT NUMBER 3: STOICHIOMETRY OF SOLUTIONS F. J. S. PRADO, A. J. H. SACCUAN, J. E. S. SERRANO, I. R. C. SIOSON School of Electrical, Electronics and Computer Engineering

In the analysis of acetic acid in vinegar under a neutralization reaction, the acidity of various brands of vinegar is determined by titrating it with standard sodium hydroxide solution using phenolphthalein as indicator. There are four brands of vinegars and all of them had been proved to be of commercial vinegar solution. Datu Puti was considered the most acidic with 3.66 percent by weight acetic acid in water. Followed by Silver Swan with 3.6%, SM Bonus has 2.52% and Amihan has 2.31%. For the analysis of dissolved oxygen in tap water under redox reaction, the amount of dissolved oxygen in a water sample is to be determined by using the iodometric or Winkler method. Calculating for the dissolved-oxygen content in tap water at normal temperature to be 7 ppm, it is determined that the tap water has the ability to maintain the survival of aquatic life. Keywords: Experiment; Chemistry; Stoichiometry; Solutions; Titration Introduction Neutralization is the reaction of an acid with the base to form a salt and, usually, water2. The significant feature of this reaction in aqueous solution is the combination of hydronium ion with hydroxide ion to form water. In this reaction, a point is reached where both acid and base are consumed and neither is in excess. This is called the equivalence point of neutralization.1 To locate the equivalence point in a neutralization reaction requires that careful control be exercised over the addition of a base to an acid (or acid to base) and a means be found to signal the point at which the reaction mixture turns from acid to base (or base to acid).. The first objective can be achieved by dispensing one solution into the other through a burette. This device measures accurately the volumes of the solutions used. The latter is achieved by using an acid-base indicator, which changes color when the solution changes from having a very slight excess of acid to having a very slight excess of base. This analytical technique of accurately measuring the volume of a solution required to react with another solution is called titration. Titrant or standard solution is the solution of known concentration while the analyte is the solution of unknown concentration. The point at which the indicator changes color is the endpoint of titration, which can differ slightly from the equivalence point.1 In this experiment, the acidity of several brands of vinegar is to be determined by titrating it with standard sodium hydroxide solution using

phenolphthalein as indicator. Vinegar is a solution that is 4-6 percent by weight acetic acid in water.1 The oxygen normally dissolved in water is important to fish and other aquatic organisms. Certain pollutants deplete the dissolved oxygen during the course of their decomposition. This is true for many organic compounds that are present in sewage or dead algae. These are decomposed by microorganisms, which use these organic compounds for food. The metabolic process is the oxidation of the organic compounds – the dissolved oxygen is the oxidizing agent. Thus while these microorganisms are removing the pollutants, they are also removing the dissolved oxygen that otherwise would be present to support aquatic life. Since the solubility of gases in solution decreases as the temperature of the solution increases, thermal pollution also decreases the dissolved oxygen content. Lost dissolved oxygen can be replenished as atmospheric oxygen usage exceeds the rate at which the oxygen can be supplied, the body of water may become anaerobic. As a logical consequence of this, one empirical standard for determining water quality is the dissolved-oxygen content (DO). The survival of aquatic life depends upon the ability of water to maintain certain minimum concentrations of the vital dissolved oxygen. At normal temperature, the maximum amount of oxygen that can possibly dissolve in water is about 9mg/L or 9ppm. For a diversified warmwater biota, the DO concentration should be at least 5ppm.1 In this experiment, the amount of dissolved oxygen in a water sample is to be determined by using the iodometric (or Winkler) method. This

procedure is based on the use of manganous compounds that are oxidized to manganic compounds by the oxygen in the water sample. The manganic compound in turn reacts with potassium iodide to produce iodine. The released iodine is then titrated with standardized sodium thiosulfate using starch as the indicator.1

sodium hydroxide used was recorded. Another trial was made. The last subpart contains the process for computing the percent by weight of acetic acid in the vinegar. The number of moles of sodium hydroxide used in the titration was computed first using equation 1.

Experimental Section (1) Chemicals. This experiment used 65 mL of 0.2M sodium hydroxide, 6 drops of phenolphthalein, 65 mL of 0.025M sodium thiosulfate, 15 mL of 1M hydrochloric acid, 10 mL manganous sulfate solution, 10 mL of alkaline iodide reagent, 10 mL vinegar samples, 6 mL of 1% starch solution and 10 mL of sulfuric acid. Apparatus. The two parts of the experiment used a base burette, burette clamp, iron stand, two 150-mL beaker, 10-mL graduated cylinder, 125-mL Erlenmeyer flask, 3 measuring pipettes, acid burette, 100-mL graduated cylinder, 500-mL Erlenmeyer flask, 500-mL plastic container, and a rubber bulb. Computation of the Percent by Weight of Acetic Acid in a Vinegar Sample. In the first part, the AcidBase Reaction: Analysis of Acetic Acid in Vinegar, consist of three subparts. These are the preparation of burette for use, analysis of acetic acid in vinegar, and the computation of the percent by weight of acetic acid in the vinegar. The first of the three subparts was already done by the laboratory assistant. This was of cleaning the base burette with a soap solution. In the analysis of acetic acid in vinegar, the previously cleaned base burette was rinsed with approximately 5 milliliters of 0.2 molar sodium hydroxide solution, making certain that no water droplet adhered to the inside wall of the burette. Some of the solution was allowed to run through the tip of the burette. This used sodium hydroxide solution was discarded in a beaker. The rinsing process was repeated two more times. The burette is then clamped to an iron stand. It was completely filled with the 0.2M sodium hydroxide solution and the air from the tips was removed by running out some of the solution into the beaker used for washings. A 5 mL of vinegar was measured and placed in a 125-mL Erlenmeyer flask. 20 mL of distilled water and 2 drops of phenolphthalein was added. The solution was mixed by swirling. The sodium hydroxide solution was added slowly while the contents of the flask were being swirled gently. The titration process was stopped when the color of the vinegar changes to light pink. The volume of the

The number of moles of acetic acid in the vinegar was also calculated using equation 2. (2) The weight of acetic acid in the vinegar and the weight of the vinegar were determined using equations 3 and 4 respectively, assuming the density of vinegar is 1 g/mL.

The percent by weight of acetic acid in the vinegar was finally calculated using equation 5.

Lastly, the average percent by weight of acetic acid in the vinegar from the two trials was computed.

Computation of the Dissolved Oxygen Content of a Water Sample. The second part, the Redox Reaction: Analysis of Dissolved Oxygen in Tap Water consists of four subparts. For the first and second procedural subparts, a 500-ml plastic bottle container was filled with tap water without any air inside first. Then, a 5 mL of manganous sulfate solution was added in the bottle using a measuring pipette. This was done by dipping the end of the pipette halfway of the water depth and releasing the contents of the pipette, causing an overflow of the water sample. After adding manganous sulfate solution, a 5 mL of alkaline iodide reagent was added using another pipette the same way of adding manganous sulfate solution. The bottle was carefully covered again to avoid splashing. The contents were mixed thoroughly by making two rapid inversions in the hand. A milky precipitate appeared. The precipitate was allowed to settle at the bottom of the bottle. When the precipitate has settled, a 5 mL concentrated sulfuric acid was added in the same way as before. The container was closed quickly and the

contents were mixed until the precipitate has completely dissolved. Titration is the third subpart. A burette was rinsed with 5 mL of 0.025M sodium thiosulfate solution, making sure that no water droplet adhered inside the wall of the burette. Some liquids were allowed to run out through the tip of the burette. The sodium thiosulfate solution used was discarded to the sink with plenty of water. The rinsing process was repeated two more times. The cleaned 25-mL burette was filled with 0.025M sodium thiosulfate solution. It was then attached to an iron stand using a burette clamp. A 500-mL Erlenmeyer flask was filled with 200 mL of the sample using a 100-mL graduated cylinder. 3 mL of 1% starch solution was also added. The sample was titrated with the standard sodium thiosulfate solution until the color of the sample changes from blue to colorless. The flask was swirled continuously while adding sodium thiosulfate. The volume of the sodium thiosulfate solution used was recorded. Another trial was made after. The number of moles of sodium thiosulfate used in the titration of the 200-mL portion of the water sample was calculated using equation 6. (

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The number of moles of dissolved oxygen was computed by equation 7. ⁄

(7)

The weight of dissolved oxygen was also determined and the dissolved oxygen content of the water sample in ppm was calculated using equations 8 and 9 respectively. To finish, the average dissolved oxygen content of the water sample in ppm was also computed.

Results and Discussion When the endpoint of titration was reached, the volume of the sodium hydroxide solution used in the titration process was recorded. The volume of 0.2M NaOH in the first trial was 15mL and 9mL in the second trial. The moles NaOH was determined in the first trial as 3x10-3moles while in the second trial, as 1.8x10-3moles using equation 1. The moles HC2H3O2, was obtained using equation 2. In the first trial the moles HC2H3O2 was computed as 3x10-3 moles then

1.8x10-3 moles in the second. Next, the weight of HC2H3O2 in grams was computed using equation 3.In the first trial the weight of HC2H3O2 was computed about 0.18g and 0.108g in the second. After computing the value of the weight of HC2H3O2, the weight of vinegar in grams was computed using equation 4. The weight of vinegar in grams in the first trial was computed as 5g and 5g also in the second trial. After getting the weight of vinegar in grams, the percent by weight of HC2H3O2 is computed next by equation 5. The percent by weight of HC2H3O2 in the first trial was computed about 3.6% and 2.16% in the second. The average percent by weight of acetic acid in the vinegar was computed by dividing the sum of the two values to two and the result was 2.88%. Table 1. Summary of Results for Analysis of Acetic Acid in Vinegar BRAND OF VINEGAR: SM BONUS TRIAL 1 TRIAL 2 Volume of 15mL 9mL 0.2M NaOH in mL Moles of NaOH 3x10-3moles 1.8 x103 moles -3 Moles of 3x10 moles 1.8 x103 HC2H3O2 moles Weight of 0.18g 0.108g HC2H3O2 in grams Volume of 5mL 5mL vinegar in ml Weight of 5g 5g vinegar in grams % by weight 3.6% 2.16% HC2H3O2 Average % by Weight 2.88% HC2H3O2 In the comparison of the activity of the different brands of vinegar, it can be determined in which of the four brands is the most acidic by computing the percentage by weight of the acetic acid. With the information gathered from different groups, it can be defined that Datu Puti is the most acidic among all of the vinegar because the average of its percent is 3.66%, which is a greater value compared to others. This can be shown in the table below.

Table 2. Comparison of the Acidity of the Different Brands of Vinegar GROUP NO. Brand of % by weight vinegar HC2H3O2 1 Amihan 1.8% 2 Sm Bonus 2.16% 3 Datu Puti 2.95% 4 Silver swan 3.6% 5 Amihan 2.82% 6 Sm Bonus 2.88% 7 Datu Puti 4.36% In the second part, after the sample was titrated with the standard sodium thiosulfate solution until the color changes from blue to colorless, the volume of the solution used was recorded. The volume of standard thiosulfate solution in the first trial was 7.1 mL and for the second trial it decreased to 6.9 mL. The number of moles Na2S2O3 was calculated using equation 6. For the first trial the moles Na2S2O3 computed is about 1.775x10-4 moles and for the second trial the moles O2 computed is 1.725 x10-4 moles. After computing for the moles of Na2S2O3, the next thing that was computed was the moles O2 using equation 7. Using this formula, it was computed that the moles O2 for the first and second 4.4375x10-5 moles and 4.3125x10-5 moles respectively. Weight of dissolved oxygen was determined to be about 1.42x10-3 grams in the first trial and 1.38x10-3 grams in the second trial, using equation 8. Lastly, for computing the dissolved oxygen content of the water sample in ppm, equation 9 was used. The liter sample used was 0.2 mL. For the ppm O2 of the first trial was 7.1 ppm and for the second trial was computed to be 6.9 ppm. The average dissolved oxygen content of the water sample in ppm was 7. Table 3. Summary of Results for the Analysis of Dissolved Oxygen in Tap Water Trial 1 Trial 2 Volume of water 200 mL 200mL sample in mL Volume of 0.025M 7.1 mL 6.9 mL Na2S2O3 in mL Moles Na2S2O3 1.775x10-4 1.725 x10-4 moles moles Moles O2 4.4375x10-5 4.3125x10-5 moles moles Weight of O2 in 1.42x10-3 1.38x10-3 Grams grams grams Dissolved oxygen 7.1 ppm 6.9 ppm in ppm Average DO 7 ppm Content in ppm

Conclusion In the first experiment, using the SM Bonus Vinegar sample, it was computed that the percent by weight acetic acid in water was 2.88%. This proves that the sample was certainly vinegar for the society. Close enough to the approximate concentration of vinegar which is 4-6 percent by weight acetic acid in water, it can be concluded that SM Bonus vinegar is less acidic than the norm. With comparison to the acidity of other brands of vinegars, Datu Puti was considered the most acidic with 3.66%. Silver Swan, SM Bonus, and Amihan were ranked accordingly next to Datu Puti for the more acidic vinegar. Silver Swan has 3.6%, SM Bonus has 2.52% and Amihan has 2.31%. Calculating for the dissolved-oxygen content in tap water at normal temperature to be 7 ppm, it is concluded that the tap water has the ability to maintain the survival of aquatic life. References (1) E.S. Espiritu, R.L. Ngo, N.D. Santos. General Chemistry Laboratory Manual Part 2. Philippines. 2011. (2) K.W. Whitten, R.E. Davis, M.L. Peck, G.G. Stanley. Chemistry. 9th ed. Belmont, CA, USA. 2010.