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To determine the x in the formula Fe (NH4)2(SO4)2i?? xH2O by titration against a standard solution of potassium manganate (VII) (permanganate). Theory: The experiment involves a redox reaction between potassium manganate (VII) and ammonium Potassium manganate (VII) is used in the experiment as it reacts completely and it is its own indicator. Potassium manganate (VII) solution is a strong oxidizing agent. In an acidic medium, manganate(VII) ion undergoes reduction as shown below. MnO4- (aq) + 8H+ (aq) + 5e- –> 4H2O (l) + Mn2+ (aq) Ammonium Iron (II) sulphate is a strong reducing agent because of the presence of Iron (II) ions.
Iron (II) ions can be oxidized to form Fe3+ which is yellow in color. Manganate (VII) ions are purple in color, when the MnO4- are reduced to Mn 2+, the color of the solution becomes pale pink, thus a self indicating titration can occur According to the overall reaction: MnO4- (aq) + 5Fe 2+ (aq) +8H+(aq) –> Mn2+(aq) +5Fe3+(aq)+4H2O(l) Between the titration of potassium manganate (VII) solution and ammonium iron (II) sulphate solution, The molarity of ammonium iron (II) Apparatus: Safety spectacles 25cm3 pipette Pipette filler 4 conical flasks 50cm3 burette Small funnel White tile Wash-bottle of distilled water Materials.
Solution A: ammonium Iron(II) sulphate solution 1M dilute sulphuric acid Solution B: potassium manganate (VII) Procedures: 1. The burette was filled with potassium manganate (VII) solution B 2. 25 cm3of the ammonium iron (II) sulphate solution A into was pipette into a conical flask.
3. 25 cm3of the dilute sulphuric acid was added into the conical flask filled with 25 cm3of ammonium iron (II) sulphate solution A 4. The conical flask was titrated with potassium permanganate solution B 5. The titration was stopped when a pale pink colour was formed in the conical flask 6. The titration was repeated several times to obtain consistent results.
7. Result Concentration of potassium manganate (VII) solutionTherefore the value of x is 6.Therefore the formula is Fe (NH4)2(SO4)2i??6H2O. Discussion: A redox titration involves a reducing agent and an oxidizing agent. In this experiment, ammonium iron (II) sulphate was the reducing agent and potassium manganate was the oxidizing agent. Observations: The solution in the conical flask was titrated with potassium permanganate, the solution in the flask first turn into a yellowish colour; this was because of the formation of Iron (III) ions formed during the redox reaction. According to the following equation: Fe 2+ (aq)+ e—> Fe3+(aq) The formation of Fe3+ caused the solution to turn yellow when it was titrated against the potassium manganate (VII).
Solution gradually turned into pale pink because of the formation of Mn2+ The excess Mn2+ caused the solution to turn to pale pink. According to the eqation. MnO4- (aq) + 8H+ (aq) + 5e- –> 4H2O (l) + Mn2+ (aq) Questions: 1. Explain why it is unnecessary to use any indicator in this experiment. –>This is because potassium permanganate can be its own self indicator because it can form a pale pink colour due to the formation of Mn2+ when it is being reduced 2. Explain why dilute sulphuric acid should be added to ammonium iron (II) sulphate solution before titration.
It prevents premature oxidation of iron (II) ions as hydrated ammonium iron (II) ions may react with air to form Iron (III) ions. It also ensures the complete reaction for manganate from manganate (VII) to Mn2+ 2. Suggest one possible error in the experiment –> The possible error of this experiment is that maybe the pipette was not washed properly as the same pipette was used to pipette 25cm3 of ammonium iron (II) sulphate and 25cm3 of sulphuric acid. The solution could have already reacted when pipette. Conclusion: The value of x in the formula of Fe (NH4)2(SO4)2i?? xH2O is 6.
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