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Question

In the laboratory SO3 can be prepared by

A
Burning Hydrogen Sulphide in the air
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B
Burning Sulphur in the air
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C
Both a and b
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D
By two stage pyrolysis Sodium hydrogen sulphate
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Solution

The correct option is D By two stage pyrolysis Sodium hydrogen sulphate
In most cases, whenever sulphur is burnt in air, SO2 is the product as the activation energy for the formation of SO3 is pretty high. Expensive catalysts like Platinated Asbestos cannot be used in the everyday laboratory.
But if you are eager to prepare SO3 without burning a hole through your wallet, we can employ the two step pyrolysis of Sodium Hydrogen Sulphate –NaHSO4
1. Dehydrating NaHSO4 at 315C, We get
2NaHSO4Na2S2O7+H2O
2. Cracking at 460C
The intermediate is called Sodium Pyrosulphate, which on cracking at 460C yields SO3
Na2S2O7Na2SO4+SO3
What can you comment on the oxidation state of the Sulphur atom(s) throughout this two-step synthesis? Is there a change in Sulphur's Oxidation state?

Can you think of one more way of preparing SO3? What will happen if we dehydrate Sulphuric acid using Phosphorus Pentoxide? Yes, we will get SO3.

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