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Aim: To find out the mass of Iron (II) sulphate each ‘Iron tablet’ contains Background Knowledge: ‘Iron tablets’ are prescribed to patients suffering from anaemia. They contain 200mg (0.200g) of iron (II) sulphate FeSO4 We are going to analyse iron tablets to find out how much Iron (II) sulphate they contain. The procedure we will use is a REDOX reaction, by oxidising the Fe2+ contained in the Iron (II) sulphate to Fe3+. We will use potassium manganite (VII) as the oxidising agent.
The active part of this is the manganite ion, MnO4-1. The SO4-2 in the FeSO4 and the K+ in the KMnO4 do not react, we say they are ‘spectator’ ions and so we will ignore them.
* 2 Iron tablets
* Pestle & Mortar
* 1M Sulphuric acid
* 100cm3 Volumetric flask
* Weighing scales
* Dropping Pipette
* Potassium Manganate (VII) – KMnO4
* 10cm3 Volumetric pipette
* Conical Flask
* White tile
Potassium Manganate (VII) solution
Potassium Manganate (VII) solution
Iron sulphate solution
Iron sulphate solution
1. Weigh the 2 iron tablets separately 2. Crush them firmly using the pestle and mortar until the tablets become crystals 3. Using a dropping pipette, add a few drops of sulphuric acid to the iron crystals and stir into a paste with the pestle 4. Transfer the iron solution into a volumetric flask carefully while using a funnel 5. Use the sulphuric acid to rinse out the mortar and pestle into the volumetric flask so all the Iron is collected 6. Rinse out the funnel as well
7. Add more sulphuric acid to a little before the mark, then use the dripping pipette to add the sulphuric acid up to the mark and cork it 8.
Shake the volumetric flask 9. Rinse out the burette with the KMnO4 solution then fill it up to the zero mark 10. Rinse out the Volumetric pipette with the Iron Sulphate + Sulphuric acid solution 11. Use the volumetric pipette to measure out 10cm3 of the Iron solution and pour this solution into the conical flask 12. Titrate the KMnO4 solution against the Iron solution by adding the KMnO4 solution carefully until it turns colourless 13. Measure the volume of KMnO4 solution used to complete the reaction 14. Empty the contents of the conical flask and repeat the experiment at least twice until 2 results (volume used) between 0.1 of each other are obtained.
Final Result: 0.184 +/- 0.00445 g
This is an accurate result as the range is very small
Conclusion: This experiment has shown that the Iron tablet does contain some Iron sulphate, about 0.184g in mass. We know this as during the titration, the Fe+2 in the FeSO4 got oxidised to form Fe+3 due to the oxidising effect of the MnO4-1. This caused the simultaneous reduction of MnO4-1, physically seen as the colour change from purple to colourless. However, the actual mass of Iron sulphate in the tablet is 0.200g. This shows that there is a small amount of error in our experiment % Error calculation: (Real result – My result/ real result) X 100 = (0.200 – 0.184/0.200) X 100
= 0.016/0.200 X 100
= 0.800 X 100
In comparison to the real result, the result obtained is fairly accurate as
the % error of 8.00% shows. An error of 8.00% shows a small difference in both values. Evaluation:
Issue| Description and effect| Improvement|
Accuracy of measurement| The measurements carried out in this experiment were very accurate as the total percentage uncertainty was +/- 4.83% (for both tablets) This figure shows that the different equipment used in this experiment were quite accurate. The burette used had an uncertainty of +/- 0.100cm3; since two readings was taken with the burette, this uncertainty doubles to give +/- 0.200cm3. In comparison to the measurement carried out (9.70cm3), the percentage uncertainty of this equipment is +/- 2.06%. This shows that the burette was very accurate as it impacted the values just a little due to the low percentage uncertainty.Another equipment used in this experiment was the 10cm3 volumetric pipette which had an uncertainty of +/- 0.04cm3. This amounts to a percentage uncertainty of +/- 0.400%. This figure also shows that the pipette used is a very accurate equipment which barely impacted our resultsAlso, a 100cm3 volumetric flask was used.
This equipment had an uncertainty of +/- 0.200cm3 and consequently a percentage uncertainty of +/- 0.200% which is also very low. This shows that the volumetric flask was very accurate.Lastly, a weighing scale was used to measure the Iron tablets. This balance had a d.p of 2.00. This means that it had an uncertainty of +/- 0.0100g and since we took the mass of 2 tablets, the uncertainty would be doubled to give +/- 0.0200g. Both tablets had a total weight of 0.920g, so the percentage uncertainty of the scale would be +/- 2.17%. This shows that the balance is fairly accurate. Although it has the largest uncertainty out of all the equipment used, it is still fairly accurate. Overall, the measurements carried out were very accurate as the low total percentage uncertainty (+/- 4.83) shows.| The accuracy of the measurements was very good; however, it could have been even better if more precise equipment was used.
For the mass of the tablets, a scale with a lower uncertainty could have been used. For example, a scale with an uncertainty of +/- 0.001g could have been used. This would have reduced the uncertainty of the masses by ten times.Also, the accuracy of the volumetric pipette could have been reduced if we used a 25.0cm3 volumetric pipette instead of a 10.0cm3 one. Although the 25.0cm3 one has a higher uncertainty value; In the bigger picture, it has a percentage uncertainty of +/- 0.24% which is 0.16% less than that of the 10.0cm3 volumetric pipette.Likewise, to improve the accuracy of the volume of Iron solution measured with the volumetric flask, we could have used a 250cm3 volumetric flask instead of the 100cm3 volumetric flask used. This would have reduced the percentage uncertainty from +/- 0.2cm3 to 0.12cm3.| Reliability of measurement| The measurements carried out in this experiment are fairly reliable.
This is can be seen as two exact values were obtained for the volume of KMnO4 solution used. This shows that the errors in the experiments were minimal.| The reliability of the measurement could be improved further if about six titrations were carried out in total and the value obtained at least thrice should be used. For example, if six titrations were carried out and the values obtained are as follow; 9.7cm3, 9.7cm3, 9.8cm3, 9.6cm3, 9.8cm3 ,9.8cm3 . The 9.8cm3 value should be selected. This would make the measurement much more accurate.| Reliability of outcome| The outcome of the experiment is very good. The % error proves this point. The % error of 8.00% shows that the experiment was set up very well and the method used really works. Also, it shows that the different equipment used fitted their different purposes. | The reliability of the outcome could have been increased by repeating the whole experiment using different equipment.
For example, change the burette and simply use a similar burette. This would reveal any errors the initial equipment might have posed. If there is a huge difference in the results obtained; e.g. obtaining the volume of KMnO4 used as 12.5cm3, use another set of similar equipment and pick the two set of experiments that have results of the highest proximity | Other Issues| When sulphuric acid was added to the crushed iron crystals, not all of the crystals dissolved completely. There were some that settled at the bottom of the container. This means that while titrating, not all the FeSO4 crystals in the conical flask were in liquid form so these would be unable to react with the KMnO4 solution. This would have reduced our results as less KMnO4 solution would be required to react with the 10cm3 iron solution as some Iron crystals haven’t dissolved.
Hence, it is a systematic error.Also, meniscus was a little difficult to see as the KMnO4’s colour made the burette markings a little difficult to see. This would be a random error as reading could be taken lower or higher than what they actually are.Also, during this experiment, we are under the assumption that the iron tablet does not contain any other compound that reacts with KMnO4 in a similar way as FeSO4 would. If there is such a compound in the tablet, it would increase the result as more KMnO4 solution would be needed to react with the other compound. This is therefore, a systematic error. While using the volumetric pipette to get 10cm3 of the iron solution, as the filled pipette was being transferred to the conical flask, a drop of iron solution dropped.
This would cause the result of that particular experiment to reduce as less KMnO4 would be required since the volume of FeSO4 available is less than what it’s supposed to be. Hence, this is a random error.| This could be improved by crushing the iron tablets using grinding machines as this would make sure the crystals are as tiny as possible making more of it to dissolve in the sulphuric acid.There is not a lot to be done to improve this as KMnO4’s colour can’t be changed. The only thing to do to reduce the effect would be to make sure the same person observes the meniscus and takes the reading as they would be able to use the same technique every time, making the error more systematic than random.There isn’t a way to improve this as we do not know all the compounds and their properties in order to know if there is or isn’t such compound.If there is a drop lost, make sure the pipette measurement is carried out again.
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