In short, two titrations have been done. The first one is the titration of acidified sodium oxalate against potassium permanganate which is used to determine the concentration of the potassium permanganate solution. The second one is the titration of hydrated oxalic acid against potassium permanganate that is used to work out the number of water of crystallization of hydrated oxalic acid.
The content of the conical flask needs to be heated to about 70°C to ensure that all the permanganate added reacts quickly. Besides, manganese(II) ions formed during the reaction has a catalytic effect and side reactions are prevented if the temperature is still maintained at above 60°C during titration.
In addition, the apparatus used has to be rinsed with deionized water, the burette solution or the pipette solution correctly to remove impurities that might affect the accuracy of the experiment.
During the beginning of the titration process, the solution in the conical flask will become dark purple even with continued swirling just after adding the first few cm3 of potassium permanganate. Sufficient time should be allowed for the reactants to mix well before manganese(II) ions formed can catalyst the reaction. Therefore, potassium permanganate should be added dropwise and slowly at the beginning.
Result
Part I
Part II
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.
This would not affect the experimental result. The dilute sulphuric acid was only used to acidify the potassium permanganate. Since the dilute sulphuric acid was neither an oxidizing agent nor a reducing agent, the acid itself would not take part in the reaction. Thus, the number of moles of potassium permanganate and acidified sodium oxalate used in the titration would not be affected.
2) For every titration in this experiment, the solution in the conical flask must be heated to about 70°C. Some water would have been evaporated and the concentration of the substance must have changed. How would this affect the experimental result? Explain your answer.
This would not affect the experimental result. Although some water would have been evaporated and the concentration of the substance would have been changed while heating, the number of moles of the substance would remain the same. Therefore, the number of moles of potassium permanganate required would still be the same.
3) Describe and explain how burette readings were taken as the burette solution, i.e., potassium permanganate, is dark purple in colour.
We usually take the burette reading from the bottom of the meniscus at eye level. However, since it was hard to observe the bottom of the meniscus due to the dark purple colour of potassium permanganate, the reading was taken from the top of the meniscus instead. This would have no effect on the accuracy of the titrations because every titrant volume is the difference between the initial and final burette readings. A piece of white paper can also be placed behind the meniscus to make the reading easier to be taken.
4) Potassium permanganate solution must be standardized before use. Explain why.
Potassium permanganate is not stable for long periods of time in storage. In the presence of light, potassium permanganate solution decomposes to give manganese(IV) oxide and oxygen gas.
4MnO4- + 2H2O → 4MnO2 + 3O2 + 4OH-
The molarity of potassium permanganate would change over time. Hence, it must be standardized before use.
5) Sodium oxalate has been used as a primary standard in this experiment. Explain what “primary standard” means and why sodium oxalate qualifies to be one.
Primary standard is a reliable, readily quantified substance. It means a substance which a standard solution of it can be prepared directly.
Sodium oxalate qualifies to be a primary standard because it is stable in air and undergoes complete and rapid reaction when used in volumetric analysis. Moreover, sodium oxalate is easily soluble in water. Thus, a standard solution of sodium oxalate can be prepared directly by dissolving a known mass of sodium oxalate in a solvent to a known volume of the solvent. Besides, it has a high molar mass so that the calculation error can be minimized.
Discussion
In part II, around 1.5g of oxalic acid crystals is weighed out. The powder is first weighed on a weighing paper before transferring to a beaker. 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
Disodium oxalate - http://en.wikipedia.org/wiki/Sodium_oxalate
Potassium permanganate - http://en.wikipedia.org/wiki/Potassium_permanganate
How to Reduce Potassium Permanganate - http://www.ehow.com/how_5243407_reduce-potassium-permanganate.html
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
