Stoichiometry Lab Report

Stoichiometry of Reactions Jessica Scanlan

Experiment No. 7 CHMY 142-25

10/14/14 Trenton Hopkins

I. Introduction

The purpose of this experiment was to examine how the stoichiometry, “the quantitative relationships between substances involved in a chemical reaction”, can be applied to determine the quantity of sodium hypochlorite found in a bleach product. This experiment allowed it to determine how much oxidizing agent is in a cleaner by using a redox reaction, which is a reaction involving the transfer of electrons from the compound being oxidized to the compound being reduced. To determine the amount of oxidizing agent, it is necessary to accurately measure out known amounts of redox reactants, know the stoichiometry

of the reaction, and have a means to determine the endpoint of the reaction. In this experiment, a titrant and titrand are mixed, then an indicator is added to the solution to give a noticeable physical or chemical change when the reaction is complete, usually a change in color. To determine the amount of NaOCl in the cleanser, a known amount of cleanser is dissolved and the solution is acidified. Iodine gives the solution a dark brown color, and then as iodine is consumed in the titration with thiosulfate, the solution turns a pale yellow. A starch indicator was added, turning the solution a dark blue. After slowly titrating a bit more, the solution turns completely clear.
II. Procedure
Standardization of the Sodium Thiosulfate Solution:
1. Using a small beaker, fill a 50.00 mL burette just below the zero mark with the sodium thiosulfate solution. Open the stopcock wide and run out any air bubbles in the burette tip, leaving a solid column of liquid in the tip. Then read and record the liquid leven in the burette to the nearest 0.01 mL.
2. To perform the standardization, dispense 10.00 L o f.0100 M KIO3 solution using a volumetric pipette into a 125 mL Erlenmeyer flask.
3. Add 25 mL of de-ionized water and approximately 2 g of solid KI. Swirl the flask until the KI dissolves, and add 3 drops of 1 M H2SO4 and mix by swirling. A deep brown color should appear. Place the flask on the magnetic stirrer and add the stir bar. Start the stirrer on a medium stir. Slowly add the sodium thiosulfate solution from the 50.00 mL burette until the brown fades to a pale yellow. Then add 6 drops of starch indicator and continue the titration slowly until the solution turns clear.
4.

Record the volume of the sodium thiosulfate solution used in the titration, and repeat the procedure in a duplicate titration.

Using the Volumetric Pipette:

1. Compress the safety bulb, hold it firmly against the end of the pipette. Then release the bulb and allow it to draw the liquid into the pipette.

2. When the liquid level is above the calibration line on the pipette, remove the bulb quickly and put your thumb or index finger over the pipette. Carefully “roll” finger to the side and allow the liquid to drop until the meniscus is level with the mark. Then hold the pipette over the flask to receive the liquid and remove the finger. Allow the liquid to drain out.

Determination of the Oxidizing Capacity of a Household Cleaner:

1. Place a clean, dry 125 mL Erlenmeyer flask on balance, and slowly dispense liquid bleach until there is about .5 g. Record the mass of bleach, and add 25 mL of de-ionized water and about 2 g of KI. Swirl contents until the KI dissolves. Then add 3 drops of 1 M H2SO4, mix, and let stand for 1 or 2 minutes.

2. Then titrate with the sodium thiosulfate solution as in the standardization procedure, adding 6 drops of starch indicator near the end of the titration. Record the volume of thiosulfate solution used in the titration. Make a duplicate

titration.
III. Safety
1. Potassium Iodate (1)- white crystalline powder, strong odor. Irritant to body tissues and moderately toxic by ingestion. Call a physician and seek medical attention for further treatment. If ingested, call a physician or poison control immediately.
2. Potassium Iodide (2)- White crystals, granules, or powder. Odorless. Avoid all body contact. If ingested, rinse mouth and give water for dilution. Call a physician or poison control immediately. Do not induce vomiting.
3. Sodium Thiosulfate Solution (3)- colorless, odorless, aqueous solution. Not considered hazardous. If ingested, give large quantities of water for dilution. Call a physician after first aid.
4. Sulfuric Acid (4)- colorless, odorless, oily liquid. Highly toxic by ingestion and inhalation. Severely corrosive to eye, skin, and other body tissues. Avoid all body contact. If ingested, do not induce vomiting and give plenty of water for dilution. Call a physician immediately.
5. Bleach (5)- extremely corrosive. May cause severe irritation to skin, eyes, and respiratory tract. If ingested, give large amounts of water and call a physician or poison control immediately. If skin contact, wash thoroughly.
6. Starch Indicator (6)- slightly turbid solution, may cause irritation to eyes and skin. Contains salicylic acid, which can be absorbed through skin. If ingested, do not induce vomiting. Call a physician or poison control.
IV. Data and Observations
Table 1: Standardization of Na2S2O3 and Titration of Bleach
Volume of KIO3 used (mL) 10
Concentration of KIO3 (M) .0100
Standardization 1 Standardization 2
Initial Volume of Na2S2O3 (mL) 47.92 49.60
Final Volume of Na2S2O3 (mL) 33.50 35.39
Volume used of Na2S2O3 (mL) 14.42 14.21
Titration 1 Titration 2
Initial Volume of Na2S2O3 (mL) 49.95 49.67
Final Volume of Na2S2O3 (mL) 29.90 37.75
Volume used of Na2S2O3 (mL) 20.05 11.92
#1 #2
Mass of Liquid Bleach Used (g) .57 .50

Observations:
In this lab, the first observation was a deep brown color found when de-ionized water, potassium iodide, and the sulfuric acid are mixed. As the titration continued, the color faded from a deep brown to a pale yellow. Once the color was a very pale, but still distinct yellow, the starch indicator was added. The starch indicator turned the solution a very deep blue/purple. More of the sodium thiosulfate solution was slowly titrated into the titrand solution, turning it from a deep blue into a clear solution. This was true for all titrations of the experiment.

Errors:
Errors in this experiment were most likely human error. A slight misreading of the volume in the burette could have caused a change in the final volume of solution used. Misreading the pipet or the de-ionized water could have also caused an error.

V. Calculations and Results
Table 2: Calculations and Results of Experiment
Standardization 1 Standardization 2
Concentration of Na2S2O3 (M) .0416 .0422
Average Concentration of Na2S2O3 (M) .0419
Titration 1 Titration 2
Moles of NaOCl (mol) 4.2*10-4 2.5*10-4
Mass of NaOCl (g) .0313 .0186
Mass percent NaOCl in sample, %(m/m) 5.5 3.7
Average Mass Percent %(m/m) 4.6

Calculations:

Concentration of Na2S2O3= (volume KIO3 used)*(Concentration of KIO3)*(3 mol I2/ 1 mol KIO3)*(2 mol Na2S2O3 / 1 mol I2)
Volume HCl= (.010 L)*(.010 M)* (3 mol I2/ 1 mol KIO3)*(2 mol Na2S2O3 / 1 mol I2)= 6.00*10-4 mol Na2S2O3

Molarity of Sodium Thiosulfate= (moles of Sodium Thiosulfate)/ (Volume (L) used in titration)
Molarity of Sodium Thiosulfate= (6.00*10-4 mol Na2S2O3)/ (.01442 L)= .0416 M

Average Concentration of Na2S2O3= (Standardization 1 + Standardization 2)/2
Average Concentration of Na2S2O3= (.0416 + .0422)/2

Moles NaOCl Used= (Vol. Na2S2O3 used (L))*(Avg. Conc. Na2S2O3 (M))*(1 mol I2 / 2 mol Na2S2O3) *(1 mol NaOCl/1 mol I2)
Moles NaOCl Used= (.02005 L)*( .0419 M)*(1 mol I2 / 2 mol Na2S2O3) (1 mol NaOCl/1 mol I2)= 4.2*10-4

Mass of NaOCl (g)= (moles NaOCl used)*(molar mass NaOCl)
Mass of NaOCl (g)= (4.2*10-4 mol)*(74.439 g/mol)= .0313 g

Mass Percent NaOCl in Sample= (grams NaOCl in sample/grams sample)*100%
Mass Percent NaOCl in Sample= (.0313 g/.57 g)*100%= 5.5%

Average Mass Percent= (Titration 1 + Titration 2)/2
Average Mass Percent= (5.4856+3.7178)/2= 4.6%

VI. Discussion of Results
In this experiment, the average concentration of the sodium thiosulfate solution was .0419 M. The standardization concentrations were very close to each other, with a difference of only .0006 M. However, the moles of the NaOCl differed greatly, which resulted in a little bit of a difference in the mass percent of NaOCl in the sample. The difference was approximately 1.8%. This error could have been a result of misreading the burette, or weigh scales while weighing the bleach or the potassium iodide. Misreading of any parts of the experiment could have changed the students calculations greatly.

VII. References
1. Inc., F.S. (2014, March 21). Potassium Iodate MSDS. Retrieved October 15,2014, from Flinn Scientific: http://www.flinnsci.com/Documents/SDS/OP/PotassiumIodate.pdf
2. Inc., F.S. (2014, March 21). Potassium Iodide MSDS. Retrieved October 15,2014, from Flinn Scientific: http://www.flinnsci.com/Documents/SDS/OP/PotassiumIodide.pdf
3. Inc., F.S. (2014, January 16). Sodium Thiosulfate Solution MSDS. Retrieved October 15,2014, from Flinn Scientific: http://www.flinnsci.com/Documents/SDS/S/SodiumThiosulfateSol.pdf
4. Inc., F.S. (2014, March 25). Sulfuric Acid Solution MSDS. Retrieved October 15,2014, from Flinn Scientific: http://www.flinnsci.com/Documents/SDS/S/SulfuricAcidSol1M-Less.pdf
5. Inc., F.S. (2014, March 25). Sulfuric Acid Solution MSDS. Retrieved October 15,2014, from Flinn Scientific: http://www.flinnsci.com/Documents/SDS/S/SulfuricAcidSol1M-Less.pdf
6. Clorox Regular Bleach, EPA Reg. No. 5813-50, [online], The Clorox Company, Oakland CA, 2009, http://www.thecloroxcompany.com/downloads/msds/bleach/cloroxregularbleach0809_.pdf, accessed 10/15/14
7. Inc., Lab Chem (2009, October 30). Starch Indicator Solution MSDS. Retrieved October 15,2014, from Lab Chem: http://www.daigger.com/Portals/0/Literature/251801_MSDS6_LABCHEM.pdf
VIII. Additional Questions
1. A new cleaning product called SuperKleenz has an oxidizing agent of unknown composition.