Applied Science BTEC Nationals/Chemical Laboratory Techniques/Iron oxalate
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The experimental details given here are given in good faith and are believed to be safe and workable methods. However, the authors cannot take responsibility for the consequences of performing these experiments.
The experiments are written for experienced science teaching staff to use as instructions for a supervised class of students. The experiments are not designed for students or inexperienced members of the public to perform without supervision. If you wish to attempt the experiments, ensure that you have completed a legally adequate risk assessment beforehand and that you work within the constraints of the risk assessment.
All ethanedioates are toxic. Wear gloves when handling these compounds. Put surplus ethanedioate in the beaker provided. Disposal of ethanedioate is achieved by adding acidified potassium manganate (VII) and warm water, and then running to waste with a large volume of water.
Preparation of iron (II) ethanedioate
Propanone is highly flammable and should not used near sources of ignition.
When a solution containing Fe+2 ions is treated with a slight excess of a solution containing ethanedioate ions (C2O4-2, otherwise known as oxalate ions), a bright yellow solid is precipitated. This compound is iron (II) ethanedioate hydrate, FeC2O4 • 2 H2O.
Fe(NH4)2(SO4)2 (aq) + Na2C2O4 (aq) + 2 H2O (l) →
FeC2O4 • 2 H2O (s) + (NH4)2SO4 (aq) + Na2SO4 (aq)
1. To 50.0 cm3 of 0.200 mol dm-3 iron (II) ammonium sulphate (Fe(NH4)2(SO4)2) add a slight excessa of 0.2 mol dm-3 sodium ethanedioate solution, with steady stirring.
2. Allow the solid precipitated to settle and test the supernatant liquid to check that precipitation is completeb.
3. Vacuum-filter the precipitate, wash with a little propanonec, and suck as dry as possible at the pump.
4. Carefully dry the precipitate at 100 C and then weigh itd.
a What volume of sodium ethanedioate should you use for a slight excess?
b How can you test the precipitate to make sure that no more iron is left in the solution?
c Propanone boils at 56 C and is miscible with water in all proportions. Suggest why the solid precipitate is washed with propanone and why this is effective.
d How do you know that the sample is completely dry?
From the mass of your final sample, calculate the percentage yield.
Analysing The Iron (II) Ethanedioate Sample
Iron (II) ethanedioate is oxidised by potassium manganate (VII) at 70 oC to give a number of products:
6 KMnO4 + 10 FeC2O4 + 24 H2SO4
6 MnSO4 + 20 CO2 + 24 H2O + 5 Fe2(SO4)3 + 3 K2SO4
This reaction can be used to titrate iron (II) ethanedioate to establish how many moles we have.
1. Weigh your pure, dry sample of iron (II) ethanedioate.
2. Precisely weigh three 0.2 g samples of iron (II) ethanedioate into three separate conical flasks.
3. Record the mass of each sample.
4. Add about 50 cm3 of 1 mol dm-3 (‘bench’) sulphuric acid to each sample flask.
5. Warm to about 70 C.
6. Titrate the warm samples with a burette filled with 0.02 mol dm-3 KMnO4.
It is important to maintain the temperature at above 70 oC to ensure that the ethanedioate ions react completely. Monitor the temperature and add boiling deionised water to the flask if necessary.
For each titration calculate:
1. How many moles of KMnO4 were used (concentration x volume)
2. How many moles (what amounts) of FeC2O4 were present (According to the balanced chemical equation, 6 moles of KMnO4 will react with 10 moles of FeC2O4).
3. What is the molar mass of FeC2O4.xH2O (mass divided by amount)
43. How many moles of water were present (the molar mass of FeC2O4 is 143.87 g mol-1 and the molar mass of one mole of water is 18.02 g mol-1).
Evaluation: How close is your answer to the correct answer?
What errors can you identify? Can you quantify these errors?
The main error is probably related to the purity of your sample.
Calculate the percentage yield of iron (II) ethanedioate:
What mass of iron (II) ethanedioate did you make?
How many moles (what amount) of iron (II) ions are present in 50 cm3 of 0.2 mol dm-3 solution?
What is the mass of this amount of iron (II) ethanedioate?
Express the mass you made as a percentage of the theoretical mass. This is the percentage yield.