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Redox titration

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Introduction

Date: 3rd December, 2010 Name: Merish Farooq Experiment no.3 Title: A redox Titration between Manganate (VII) and Iron (II) Objective: To determine the x in the formula Fe (NH4)2(SO4)2�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) ...read more.

Middle

2.46� 10-3 By the finding the no of moles of Fe 2+, we can find the Molarity by using the formula, Molarity = No of moles Volume Molarity of Fe 2+ = 2.46� 10-3 25/1000 = 0.0985 mol dm-3 By finding the Molarity we can find the mass of anhydrous Fe (NH4)2(SO4)2 in one dm3. Molar mass of Fe (NH4)2(SO4)2 = 55.8+ (14.0+4) + (32.1 + 16�4) �2 = 55.8+ 36+192.2 = 284 g mol Molarity of Fe 2+ = molarity of Fe (NH4)2(SO4)2 Therefore, Molarity of Fe (NH4)2(SO4)2 = 0.0985 mol dm-3 The No of moles of Fe (NH4)2(SO4)2 can be found by the formula: Molarity � Volume = no of moles 0.0985 � 1 dm-3 = 0.0985 moles Therefore the no of moles of Fe (NH4)2(SO4)2 is 0.0985 moles. The mass of anhydrous Fe (NH4)2(SO4)2 can be found by: 0.0985 mol � 284 g mol = 27.97g Therefore the mass of anhydrous Fe (NH4)2(SO4)2 in 1 dm-3 is 28 g. ...read more.

Conclusion

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)2�xH2O is 6. ...read more.

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Response to the question

There is a clear aim in this piece of coursework, and the student has structured their essay in a logical order, going into suitable depth where necessary. The response to the question is good, they have given a brief introduction ...

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Response to the question

There is a clear aim in this piece of coursework, and the student has structured their essay in a logical order, going into suitable depth where necessary. The response to the question is good, they have given a brief introduction of the task and then gone through all the stages, methodically, to include all the information necessary to reach their conclusion and answer. The calculations section is all correct, however, it is very long to read and could have been broken down into several sections to make it easier to understand.

Level of analysis

The student suggests understanding of the topic throughout the essay and clearly knows the theory behind the experiment well. The correct reaction equations are used and the calculations are all correct, with full working out, leading to the right answer/conclusion. It is good to include a discussion section, but this one is a little too brief. Specific examples for this candidate to improve could be to clearly define what a reducing/oxidising agent is to show understanding. They could also have talked more about possible experimental errors and how they worked to reduce them.

Quality of writing

In terms of spelling and punctuation, the essay was fine. There were a few moments in which the grammar could have been better; for example 'solution, The', there should not be a capital letter in the middle of a sentence. The structure of the essay was fine, however, it would have perhaps been better if the conclusion came before the questions, as then it links with their experiment a little better.


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