Analyzing Bleach in a Redox Titration

I. Purpose
The purpose of this lab was to use redox titration to determine the strength of a household bleaching solution and, thus, gain experience with analytical procedures and expand our knowledge of redox reactions and titration calculations. We accomplished this by…

II. Procedure
1. Fill one berel pipet with bleach (sodium hypochlorite solution) and one with sodium thiosulfate. Weigh and record the mass of each. (Each solution has a density of 1 g/mL.)
2. Place seven drops of bleach into a 50 mL Erlenmeyer flask. Reweigh and record the mass of the bleach pipet. Calculate the mass and volume (mL) of bleach used. Record these values.
3. Add 14 drops of 1M KI solution to the bleach. Then add 7 drops of 6M HCl to the bleach and mix it thoroughly by swirling. The resulting solution will be dark brown with some particles of matter.
4. Add drops of sodium thiosulfate to the bleach until the dark color lightens to amber. Then add 2 or 3 drops of starch indicator; the presence of iodine will make the solution dark blue. Continue adding sodium thiosulfate one drop at a time until the solution becomes colorless. Reweigh and record the mass of the sodium thiosulfate pipet. Calculate the mass and volume (mL) of sodium thiosulfate used. Record these values.
5. Repeat the titration for a second sample.

III. Observations
In the beginning solution (bleach and KI), brown chunks precipitated. As sodium thiosulfate (Na2S2O3) was added, the brown color changed to a lighter amber hue. The starch indicator created an ugly forest green color. The solution turned clear after 1-2 drops of sodium thiosulfate were added.
IV. Equations and Chemical Formulas
Cl2 + 2OH-1 (aq) -> ClO-1 + Cl-1 + H2O
ClO-1 (aq) + 2I-1 (aq) + 2H+1 (aq) -> I2 (aq) + Cl-1 (aq) + H2O
OX: 2I-1 -> I20 + 2e-
RED: Cl+1 + 2e- -> Cl-1

I2 (aq) + 2S2O3-2 (aq) -> 2I-1 (aq) + S4O6-2 (aq)

Mol = Volume (L) * Molarity (M)
Mol of Na2S2O3 Solution: L Na2S2O3 * Molarity Na2S2O3
Molarity of Na2S2O3 Solution: Moles of Na2S2O3 / 1 L
Molarity of Na2S2O3 Solution: 0.55M
Sodium Thiosulfate / Bleach Mole Ratio: 2 Na2S2O3 / 1 NaClO
Average Molarity of NaClO: ½(Trial 1 Molarity + Trial 2 Molarity)
Mass (g) of NaClO in Bleach Sample: Mol NaClO * Molar Mass NaClO
Mass % of NaClO in Bleach Sample: 100 * (Mass NaClO in Bleach Sample / Mass of Bleach Sample Used)
Average Mass % of NaClO: ½ (Trial 1 Mass % + Trial 2 Mass %)
Molar Mass of NaClO: 74.5g/mol
Density of Na2S2O3 Solution and of Bleach Solution: 1 g/mL
1 L: 1000 mL

V. Data and Calculations
Data
Category Trial 1 Trial 2
Initial Mass (g) of Bleach Pipet 3.48g 4.11g
Mass (g) of Bleach Pipet After Using 7 Drops 3.19g 3.86g
Initial Mass (g) of Sodium Thiosulfate Pipet 3.57g 3.75g
Mass (g) of Sodium Pipet After Titration 2.78g 2.95g

Calculations
Category Trial 1 Trial 2
Mass (g) of Bleach Used in Titration 0.29g
(3.48g – 3.19g) 0.25g
(4.11g – 3.86g)
Volume (mL) of Bleach Used in Titration 0.29mL
(1g/mL) 0.25mL
(1g/mL)
Mass of Na2S2O3 Solution Used in Titration 0.79g
(3.57g – 2.78g) 0.8g
(3.75g – 2.95g)
Volume (mL) of Na2S2O3 Solution Used in Titration 0.79mL
(1g/mL) 0.8mL
(1g/mL)
Moles of Na2S2O3 0.0004345 mol
(0.55M * 0.00079L) 0.00044 mol
(0.55M * 0.0008L)
Moles of NaClO 0.0002173
(0.0004345 mol / 2) 0.00022 mol
(0.00044 mol / 2)
Molarity of NaClO (Bleach Solution) 0.7491379M
(0.0002173 mol / 0.00029L) 0.88M
(0.00022 mol / 0.00025L)
Average Molarity of NaClO 0.814569M
0.7491379M +0.88M
2
Mass (g) of NaClO in Bleach Sample 0.16185g
(0.0002173 mol * 74.5g/mol) 0.01639g
(0.00022 mol * 74.5g/mol)
Mass % of NaClO in Bleach Sample 5.58%
100(0.16185g / 0.255g) 6.56%
100(0.01639g / 0.25g)
Average Mass % of NaClO 6.07%
0.5(5.58% + 6.56%)

VI. Summary
In this lab, we learned how to determine the strength of a household bleaching solution through redox titration. It was a most excellent experiment; if I knew what the mass % of the solution has to do with its relative strength, I would be able to make many housewives happy. Nevertheless, I am pleased that I gained experience in the laboratory.
This lab was performed successfully. This was the first time we did a lab with the same team that we had the lab before it. It was a pleasant experience, but it didn’t change much. We still got the lab done.
I feel a little remorse about finishing this lab report, because it is my last. Yet this also pleases me. I have worked hard this year and suffered more than my share of bad luck. Here’s hoping that I can get some good luck in time for this final.
Mr. Mylin, I think Katie will set the state record next year and sectionals, but that’s just a guess on my part. She works hard. She likes running. Ask her about her English Final Project. It was really cool.

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