Redox titration of copper evaluation

Redox Titration of Sodium Thiosulphate against Copper (II) Sulphate

My results from this titration can be seen in the table below:

The concordant results I will be using in my calculation are the titres from my 2nd and 3rd titration as there are within 0.01 cm3 of one another. To calculate the average titre I will use the following method:

Average Titre = (24.20 + 25.25)
2
= 24.225 cm3

In this titration there are two half equations that are involved, the first is when the copper (II) sulphate is added to the potassium iodide:

Password

Confirm Password

I agree to the terms of service and privacy policy

2Cu2+ + 4I- → 2CuI + I2

The second half equation takes place when the sodium thiosulphate is titrated into the solution containing the Cu2+ and iodine:

I2 + 2S2O32- → S4O62- + 2I-

As I need to find the concentration of the Cu2+ I have to look at both half equations to find the ratio of Cu2+ to S2O32-, which is 1:1. This is because in each step of the reaction there are 2 moles of each. This will mean that the number of moles of Cu2+ will be the same as the number of moles of S2O32-:
...

This is a preview of the whole essay

2Cu2+ + 4I- → 2CuI + I2

The second half equation takes place when the sodium thiosulphate is titrated into the solution containing the Cu2+ and iodine:

I2 + 2S2O32- → S4O62- + 2I-

n of S2O32- = 0.102827763 x 24.225
1000
= 2.491002571 x 10-3 mol

n of Cu2+ = 2.491002571 x 10-3 mol

Now that I have the number of moles of Cu2+ I can calculate the concentration of it:

c = n
v

c of Cu2+ = 2.491002571 x 10-3
(25/1000)

= 0.099640102 mol dm-3

I now need to calculate the percentage uncertainties associated with the apparatus I have used. From that I can then calculate the precision error involved in this experiment:

% uncertainty of the 25cm3 pipette = 0.06 x 100 = 0.24 %
25.0

% uncertainty of the 50cm3 burette = 0.10 x 100 = 0.342759211 %
29.175

Therefore the total uncertainty = 0.582759211 %

So, to work out the percentage error I need to work out the percentage of the concentration that could be incorrect:

Percentage Error = Total uncertainty x Concentration
100
= 0.582759211 x 0.099640102 mol dm-3
100
= ± 5.806618723 x 10-4

Now that I have the percentage error I can show the possible errors that there could be in the calculation of the concentration of Cu2+:

Highest possible c of Cu2+ = 0.100220763 mol dm-3
Lowest possible c of Cu2+ = 0.09905944 mol dm-3

However I will represent the final concentration I have calculated of the Cu2+as:

0.099640102 ± 5.806618723 x 10-4 mol dm-3
0.0996 ± 5.81 mol dm -3 (3sf)

Peer Reviews

Here's what a star student thought of this essay

skatealexia

8th of Jul 2012

Quality of writing

The candidates spelling, grammar and punctuation are all fine, they highlight their final answers in bold which will also make the answers clear and easier to mark.

Level of analysis

The decimal places the candidate uses is consistent in the table which means that the candidate could make accurate calculations from them. They also repeated the titrations four times and started off with a trial meaning their results have a greater chance of being reliable. The number of decimal places presented in the end result should ideally be consistent with the number of decimal places used in the table, and in this instance it is not. The candidate does calculate percentage errors which is good to know the margin of possible error in the experiment and how this may affect it, but they do not explain the significance of the results.

Response to question

Overall a good example of calculations needed for experiments. The candidate titles this essay as an 'evaluation,' but rather than an evaluation being present, there is just a set of calculations. For an evaluation the candidate should have included possible errors that might have gone wrong with the experiment and how this could have been improved. The candidate should have displayed the method used so that they could have shown where they got this information from about possible errors in a concise format. To do this sort of evaluation displays a higher understanding of every aspect of the practical and these are important skills to develop for the chemistry industry.

Did you find this review helpful? Join our team of reviewers and help other students learn