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Determination of the Number of Water of Crystallization in Oxalic Acid Crystals

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Introduction

Determination of the Number of Water of Crystallization in Oxalic Acid Crystals Objective To determine the number of water of crystallization in a given sample of oxalic acid crystals by redox titration. There are two parts of experiment. In the first part, the determination of the concentration of a given potassium permanganate solution using a standard sodium oxalate solution by titration is expected. In the second part of the experiment, the aim is to work out the number of water of crystallization of hydrated oxalic acid based on data obtained from a titration experiment. Introduction This experiment concerns a redox titration. A given solution of potassium permanganate is standardized using a standard solution of sodium oxalate. A solution of hydrated oxalic acid is prepared and is titrated against the standardized potassium permanganate solution. An indicator is not required since a negligible excess of KMnO4 solution turns the contents of the conical flask from colourless to slight pink that marks the end-point. 2MnO4-(aq) + 16H+(aq) + 5C2O42-(aq) � 2Mn2+(aq) + 10CO2(g) + 8H2O(l) Experimental Procedure Part I Standardization of potassium permanganate 1. 25cm3 of the standard solution of Na2C2O4 is transferred into a conical flask and approximately equal volume of dilute H2SO4 is added to provide H+(aq) ...read more.

Middle

Trial 1 2 3 Final burette readings /cm3 26.79 28.48 27.59 26.94 Initial burette readings /cm3 2.36 4.32 3.31 2.82 Volume used /cm3 24.43 24.16 24.28 24.12 Mean titre /cm3 24.19 Calculation 2MnO4-(aq) + 16H+(aq) + 5C2O42-(aq) --> 2Mn2+(aq) + 10CO2(g) + 8H2O(l) 1) From result in Part (A), calculate the concentration of the given solution of KMnO4. Let M be the molarity of the given solution of KMnO4 ? The concentration of the given solution of KMnO4 is 0.0200 mol dm-3 2) From results in Part (B), calculate the number of moles of oxalic acid in 250 cm3 solution No. of moles of oxalic acid in 250 cm3 = 0.0121 mol Let M be the molarity of the oxalic acid ? The concentration of the oxalic acid is 0.0484 mol dm-3 3) Calculate the number of water of crystallization of hydrated oxalic acid, i.e., the value of n the formula H2C2O4.nH2O. Conclusion ?The formula of hydrated oxalic acid is H2C2O4.nH2O Question 1) In step 1 of part (A), the volume of sulphuric acid added was not precisely known. How would this affect the experimental result? Explain your answer. ...read more.

Conclusion

The disadvantage of this weighing method is that the sample will have contact with an intermediate container (i.e. weighing paper) and, therefore, there are chances of losing some solid in the transfer process. Thus, the oxalic acid crystals should be weighed directly in the beaker. This technique also avoids spills on the balance pan since the transfer process can be done outside the balance case. During the titration process, some potassium permanganate solution may be carelessly left in the inner wall of the conical flask while swirling. This would lead to overestimation of the volume of potassium permanganate solution required for the reactions. Thus, the burette tip should be inserted into a more inner position of the conical flask during titration. After obtaining the potassium permanganate solution, the solution was left in the beaker throughout the whole experiment. The experiment lasted for a long time and the potassium permanganate might have been decomposing slowly. This would make the molarity of the solution lower than that calculated in Part I. In order to avoid this, the potassium permanganate can be stored in a brown bottle after obtaining. Reference 1. Disodium oxalate - http://en.wikipedia.org/wiki/Sodium_oxalate 2. Potassium permanganate - http://en.wikipedia.org/wiki/Potassium_permanganate 3. How to Reduce Potassium Permanganate - http://www.ehow.com/how_5243407_reduce-potassium-permanganate.html 4. Preparation of a Standard Solution - http://eclass.ssc.edu.hk/inabox/files/c20/reference/4f6149b8730f80f37f04338f6fe7b451_1235373694AL%20Chemistry/AL%20Chemistry/Lab%20Manuals/Form%206/F6_09lab01_Standard_Solution.pdf ...read more.

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