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ANALYTICAL CHEMISTRY (CHM421)
EXPERIMENT 6 ANALYSIS OF BLEACH AND COPPER(II) UNKNOWN
STUDENTS’ NAME:
NURUL SYAFIQAH BINTI MOHD AINI
[2012697062]
NOOR SHUHADA BINTI BINTI MUSTAPA
[2012411706]
GROUP:
ASB2AC LECTURER’S NAME:
MADAM SALAMIAH BINTI ZAKARIA
DATE OF EXPERIMENT/DATE OF SUBMISSION: 18 APRIL 2013 / 2 MAY 2013
TITLE:
Analysis of bleach and copper(II) unknown
OBJECTIVES:
To standardise the Sodium Thiosulphate Solution
To determine the percentage of NaClO in bleach sample
To determine the percentage of Copper in an unknown copper sample
RESULTS:
A. Standardisation of the Sodium Thiosulphate Solution
Final volume of thiosulphate solution (mL) Initial volume of thiosulphate solution (mL) Volume of thiosulphate solution (mL)
1 15.6 0.0 15.6
B. Determination of neutralization capacity of an antacid tablet Brand of bleach 1 = Kao Bleach, Sodium Hypochlorite 5% 1. 2. 3. 4. 5.
Molarity of Na2S2O3 solution = 0.1915 M Mass of volumetric flask + 10 mL bleach = 73.3501g Mass of volumetric flask = 62.9862g Mass of 10 mL bleach = 10.3638g Density of bleach = 1.0364g/mL
2 31.6 15.6 16.0
3 47.0 31.6 15.4
Final volume of thiosulphate solution (mL) Initial volume of thiosulphate solution (mL) Volume of thiosulphate solution (mL) Moles of Na2S2O3 Moles of NaClO in 10mL aliquot Moles of NaClO in 100mL aliquot Moles of NaClO in 10mL bleach sample Average moles of NaClO in 10mL bleach sample g/L of NaClO in bleach Mass percent of NaClO in bleach Mass percent of NaClO in from
Molarity of Na2S2O3 solution = 0.1915 M Mass of volumetric flask + 10 mL bleach= 75.340g Mass of volumetric flask= 64.674g Mass of 10 mL bleach= 10.666g Density of bleach= 1.0666g/mL
Final volume of thiosulphate solution (mL) Initial volume of thiosulphate solution (mL) Volume of thiosulphate solution (mL) Moles of Na2S2O3 Moles of NaClO in 10mL aliquot Moles of NaClO in 100mL aliquot Moles of NaClO in 10mL bleach sample Average moles of NaClO in 10mL bleach sample g/L of NaClO in bleach Mass percent of NaClO in bleach Mass percent of NaClO in from
C. Analysis of copper(II) unknown Molarity of Na2S2O3 solution = 0.1915 M
Mass of copper(II) compound taken (g) Final volume of Na 2S2O3 solution (mL) Initial volume of Na2S2O3 solution (mL) Volume of Na2S2O3 solution (mL) Moles of Na2S2O3 Moles of Cu + Gram of Cu + Weight percent of copper in sample Average weight percent of copper in sample Theoretical mass percent of copper in sample Percent error Name of unknown copper(II) unknown Chemical formula of unknown copper(II) unkmown
0.877 71.61 copper (II) sulphate pentahydrate CuSO4.5H2O
DISCUSSION:
In this experiment the amount of hypochlorite ion present in a solution of bleach is determined by an oxidation-reduction titration, the iodine-thiosulfate titration procedure. In acid solution, hypochlorite ions oxidize iodide ions to form iodine, I 2. The iodine that forms is then titrated with a standard solution of sodium thiosulfate. The analysis takes place in a series of steps: (1) Acidified iodide ion is added to hypochlorite ion solution, and the iodide is oxidized to iodine. 2H+(aq) + ClO-(aq) + 2 I-(aq)
Cl-(aq) + I2(aq) + H2O(l)
(2) Iodine is only slightly soluble in water. It dissolves very well in an aqueous solution of iodide ion, in which it forms a complex ion called the triiodide ion. The triiodide ion is yellow in dilute solution, and dark red-brown when concentrated. I2(aq) + I-(aq)
I3-(aq)
(3) The triiodide is titrated with a standard solution of thiosulfate ions, which reduces the iodine back to iodide ions: I3-(aq) + 2S2O32-(aq)
3I-(aq) + S4O62-(aq)
During this last reaction the red-brown color of the triiodide ion fades to yellow and then to the clear color of the iodide ion. It is possible to use the disappearance of the color of the I3- ion as the method of determining the end point, but this is not a very sensitive procedure. Addition of starch to a solution that contains iodine or triiodide ion forms a reversible blue complex. The disappearance of this blue colored complex is a much more sensitive method of determining the end point. The quantity of thiosulfate used in step (3) is directly related to the amount of hypochlorite initiall y present. To know the copper (II) unknown is, we calculate first the average molarity of thiosulphate which is 5.667 M. from this equation : H2O + ClO- + 2S2O32-
S2O62- + Cl- + 2OH-
We can get the number of moles of ClO - . After this, we can calculate the mass of hypochlorite in an unknown copper(II). The percentage get is 25.45 % copper salts. To know the unknown copper (II), we compare with the percentage of copper (II) given. From this calculation, the nearest percentage in this experiment is suit with copper (II) sulphate pentahydrate. The difference in the percentage obtained is due to the errors that occur while conducting the experiment. The first error is while we pipette the bleach to 10mL, we not get the accurate volume needed because of incorrect reading of the meniscus level for pipette. Beside, for the repeated sample, it is possible that we forget to add starch to determine the
end point instead we just let the colour of iodine changes to colourless. The volumetric flask is not cleaned thoroughly thus causing some impurities inside the flask. This will also affect the mass of sample weight. The 3 replicate sample of copper (II) unknown is not precise with each other because it left in the weighing boat. Other error is while determining the end point of the titration. The titration process should be stopped immediately once the end point is reached. End point is when the colour of the analytes solution changes to the required colour. Any extra drop of the titrant will affect the calculation of the molarity of the iodide present in the solution. There are also error that occurs due to the improperly calibrated burettes and conical flasks causing them to have volumes slightly different from those indicated by their graduations. These apparatus should be frequently calibrated in order to obtain more accurate reading of measurement.
CONCLUSION:
The experiment is very important because it gives the idea of oxidation and reduction at the same time it views a clear about iodometric reaction. From the calculation, the percentage by mass of the NaClO in Kao is 0.00217% while in Clorox is 0.00502 %. The percentage of copper in an unknown copper sample is 25.45%. This percentage shows that the unknown copper (II) is actually copper (II) sulphate pentahydrate.
