Method:
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2cm3 of commercial bleach was pipette into a conical flask, which was then added with 20cm3 of water, 10cm3 of potassium iodide (0.5 mol dm-3) mixture.
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This was then titrated against standard sodium thiosulphate which had the concentration of 0.1 mol dm-3. The sodium thiosulphate was placed in a 50cm3 burette.
- Once solution turned blue starch indicator was then used until the iodine colour started to fade (it went from blue to colourless).
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This was then repeated until the titration values were 0.1cm3 of each other. These steps were then repeated again for the next bleach samples.
Results and Calculations:
Table of Results for Sample A:
This table shows the results gained from the experiment and the number of attempts that were done. It also shows that sample A had the titration value of 18.00cm3.
Table of Results for Sample B:
This table shows the results gained from the experiment and the number of attempts that were done. It also shows that sample B had the titration value of 7.00cm3.
Table of Results for Sample C:
This table shows the results gained from the experiment and the number of attempts that were done. It also shows that sample C had the titration value of 12.00cm3.
Table of Results for Sample D:
This table shows the results gained from the experiment and the number of attempts that were done. It also shows that sample D had the titration value of 10.00cm3.
Table of Average Results:
Here shows the table of average results, giving their mean value and also what sample each value belonged to.
To work out the mean, the titration values were added up altogether then divided by 3 since that was the amount of values that were there. This was done with each sample.
The molar concentration of each bleach solution will now need to be calculated.
Concentration for Sample A:
If the titration value = 18cm3, then 18cm3 of 0.1dm-3 of thiosulphate is equal to 18cm3 of 0.05 mol dm-3.
If 1000cm3 of a solution contains 0.05 moles of iodine, then 18cm3 of bleach would contain X amount of moles of bleach. To work this out, this formula will be used:
0.05 x T/1000
Where T is equal to the titration value.
0.05 x 18/1000
=0.9/1000
=0.009 moles of iodine
The volume of bleach used in the titration was 2cm3; this value can be used to find out the concentration of sample A:
0.05 x 18/1000 X 1000/2
=0.9/1000 x 1000/2
=0.009 x 500
= 4.5 mol dm-3 of hypochlorite was found in this sample.
Concentration for Sample B:
Since the titration value was 7cm3, then the 7cm3 of 0.1 mol dm-3 of thiosulphate is equal to 7cm3 of 0.05 mol dm-3 of iodine.
If 1000cm3 of a solution contains 0.05 moles of iodine then 7cm3 contains X moles of bleach. To work out the amount of bleach, this formula would be used:
0.05 x T/1000
Where T is equal to the titration value.
0.05 x 7/1000
= 0.35/1000
=0.00035 moles of iodine
The volume of bleach used was 2cm3, so therefore this can be used in this equation to work out the concentration of sample B:
0.05 x 7/1000 X 1000/2
0.00035 x 500 = 0.175 mol dm-3 of hypochlorite
Concentration of Sample C:
Since the titration value for this sample was 12cm3, it is safe to say that 12cm3 of 0.1 mol dm-3 of thiosulphate is equal to 12cm3 of 0.05 mol dm-3 of iodine.
If 1000cm3 of a solution contains 0.05 moles of iodine then 12cm3 of bleach contains an X amount of moles of bleach. To work this out this formula will be used:
0.05 x T/1000
Where T is equal to the titration value
0.05 x 12/1000
= 0.6/1000 = 0.0006 moles of iodine
Since the volume of bleach used in the titration was 2cm3, this value can then be used to find out the concentration of sample C:
0.05 x 12/1000 X 1000/2
= 0.0006 x 500 = 0.3 mol dm-3 of hypochlorite
Concentration for Sample D:
The titration value for Sample D was 10cm3; this means that 10cm3 of 0.1 mol dm-3 of thiosulphate is equal to 10cm3 of 0.05 mol dm-3 of iodine.
If 1000cm3 of a solution contains 0.05 moles of iodine then what 10cm3 contains will have to be worked out:
0.05 x T/1000
Where T is equal to the titration value.
0.05 x 10/1000
0.5/1000 = 0.0005 moles of iodine
Since the volume of bleach used in this sample was 2cm3, this value can now be used to work out the concentration of sample D:
0.05 x 10/1000 X 1000/2
0.0005 x 500 = 0.25 mol dm-3 of hypochlorite
The results show that sample A had the highest concentration of bleach whilst sample B had the lowest concentration of bleach.
Conclusions:
These results showed that sample A had the highest concentration of bleach, which was 4.5 mol dm-3. Sample C came second, which had the concentration of 0.3 mol dm-3 and sample C came third with 0.25 mol dm-3. Whereas sample B had the lowest concentration with 0.175 mol dm-3. I think this shows that sample A had the ‘highest value for money’ since it contained the highest concentration of hypochlorite, which is a chemical that is found in commercial bleaches. Since sample B had the lowest concentration of thiosulphate, I think this shows that this would have the lowest ‘value for money’ since it may not be good in everyday household use. It can also be said that sample B will have the lowest cost because of this.
Evaluation:
No anomalies were present during the titration experiment, this shows that the titration was carried out accurately. It also shows that the results gained can be trusted and are reliable. The reason for this could be the fact that the measurements carried out were done accurately, due to making sure that eye level was kept whilst measuring. And also that the dip was on the meniscus line each time solutions were measured. Furthermore equipment was always cleaned with distilled water to prevent contamination. In addition the same amount of drops of starch indicator (3 times) was used each time during the titration to increase the chances of getting accurate results. The funnel was also made sure to be removed once sodium thiosulphate was being added to the bleach. Also before starting the titration the tap on the burette was made sure that it was working properly and that it was rinsed out with sodium thiosulphate (since that was the chemical that was to be used in the burette).
References:
1Chemguide. 2002. pH (titration) curves [online]
Available at: [Accessed 16th November 2009]
