Analysis of a Commercial Bleach Lab: 3/29/17 PURPOSE The purpose of this lab experiment is to determine the amount of sodium hypochlorite (NaClO) in commercial bleach through titration.
PRE-LAB QUESTIONS 1.) Titration Titration is a technique where a solution of known acid or base concentration is used to determine the concentration of an unknown acid or base solution.
2.) Moles = Molarity * Volume
(0.0500M NaOH)(16.7) = x
x = .0334 M
3.) 0.0334 M * 250 mL = x * 10.0
x = 0.0334 * 250 / 10.0
x = .835
4.) 1.05 * 10 mL = 10.5 g HC
PROCEDURE
• Transferred 5.00-mL of commercial bleach solution with distilled water into a 100-mL graduated cylinder to the mark
• Weighed 2 grams of KI and swirled it into the 25.0-mL dilute bleach from an Erlenmeyer flask
• Added 2-mL of 3 M HaI while swirling the solution to get a dark yellow to red-brown color
• Titrated with 0.10 M sodium thiosulfate solution until the iodine(dark-yellow solution) faded to light yellow then added one o ne dropperful of starch solution to get a blue color
• Continued the titration until a drop of sodium thiosulfate solution causes the blue color to disappear and then recorded the final buret reading before repeating the titration procedure for a second trial
MEASUREMENTS TO BE TAKEN The initial and final buret readings with the volumes of Na2S2O3 added that had to be taken.
MATERIALS
Materials 5% Bleach, 5-mL 3 M Hydrochloric acid, 6-mL Potassium iodide, 6g 0.100 M Sodium thiosulfate solution, 70-mL 2% Starch solution, 3mL Balance Buret with Buret clamp
Materials 125-mL Erlenmeyer flask Pipet bulb Ring Stand 5-mL and 25-mL transfer pipets 100-mL Graduated cylinder Distilled water Weighing dish
DATA
Molarity of Na2S2O3
Trial 1(.100 M)
Trial 2(.100 M)
Trial 3(.100 M)
Trial 4(.100 M)
Initial Buret Reading
0.0mL
0.0mL
0.0mL
0.0mL
Final Buret Reading
24.0mL
22.0mL
24.6mL
32.6mL
Volume of Na2S2O3 added
24.0mL
22.0mL
24.6mL
32.6mL
Commerci rcial Blea leach befor fore
Comm ommerc ercial ial Bleach ach afte afterr
Comm ommercia rciall bleac each afte afterr
titration
titration with sodium thiosulfate
titrating with the starch
CALCUATIONS
RESULTS Based on the data collected and the calculations, with the already known concentration of sodium thiosulfate, the mass of NaClO in commercial bleach was approximately 21.975 mL that was used in the titration procedure, which amounts to a percent of 5.51%. Since 6% of the commercial bleach is sodium hydrochloride, the percent error was 8.17%(which is in proximity when compared to the original percent error being 8.25%).
ANALYSIS While the overall point of this lab was to find amount of sodium hypochlorite in commercial bleach, the lab was able to teach our group how sodium thiosulfate and starch really affected our solution based on the equations given from the background information. Our scientific error that influenced our data was using diluted water to clean out our solution’s reminded droplets from the graduated cylinder. The H2O affects our solution when titrated with the sodium thiosulfate as well as the starch droplet and thus affects our mass of NaClO with the percent error increased more due to this. This lab was able to work because it uses titration in order to make the solution more accurate than applying it through a pipet. A change that our group would’ve done to make our data more accurate was with the 0.10 M sodium thiosulfate solution, remembering how many drops we used until the iodine faded to light yellow. We would also inform the other group, that we partnered with, in order to see how accurate the sodium thiosulfate solution would be in making the solution from dark-yellow to light-yellow. The lab is worth repeating if our group and our partnered group shared the same sodium thiosulfate solution instead of creating an individual group solution, the perhaps the percent error would’ve been on or closer to 8.25%.
POST-LAB QUESTIONS 1.) Oxidation is defined as an increase in oxidation number, and reduction as a decrease in oxidation number. This is achieved through the transfer of electrons. In oxidation, the reductant yields the product plus an electron. In reduction, the oxidant plus electron yields a product.
2.) 2 H+ + ClO! + e!
2 I!
Cl! + H2O reduction
!
I2 + e! oxidation
!
this equation shows iodide is being oxidized and the hypochlorite ions are being reduced.
I! + I2 + 2 e!
2 S2O3 2!
3 I! reduction
!
S4O6 2! + 2 e! oxidation
!
this equation shows the thiosulfate ions are being oxidized and iodine is being reduced.
3.) Using an aliquot in diluting the original solution is an advantage because it makes the titration more effective by conserving both time and materials. When the substance is diluted, only a small portion is used, so there will be less moles present in the solution. So it’ll take less time to neutralize. Due to the lower number of moles present, the color change, from dark yellow to a light yellow, occurs much faster.
4.) Four 25.0 mL aliquots can be measured from a 100 mL volumetric flask, but only in theory. When put in practice, not exactly 25.0 mL of solution can be extracted from the volumetric flask due to inaccuracy errors.
5.) Each oxygen has an oxidation number of -2, with an overall charge of -12. This molecule has a charge of -2, meaning that the total charge of the sulfurs must equate to +10. A 10+ charge does not divide evenly into 4 sulfur atoms so the charges aren’t all equal. The charges would likely be +2, +2, +3, +3. Overall, no I don’t believe that all the sulfur atoms have the same oxidation numbers.
6.)
a.) The calculated percentage will be less than the expected value cause the distilled water in the pipet will dilute the commercial bleach solution to an extent. So then the number of moles will also decrease and when divided by the total mass of the bleach, a smaller percent of NaClO in the bleach is calculated.
b.) Adding an extra gram of KI will not show any change in the calculated percentage of NaClO because KI is already added in excess in the procedure.
c.) In this experiment, less solid iodine would cause an indirect reaction with sodium thiosulfate and therefore a smaller calculation in the number of moles of NaClO will be determined and a smaller percentage will result.
7.) This question is answered in ANALYSIS
