Question

why conical flask is fitted with a cork

Answer 1

So that while perfoming the experiment the air does not move in and out of the flask disturbing the experiment. Cork acts as a barrier to the flow of air.

Answer 2

it is used to make sure that no reactant or product leaves the reaction system, and also no outside substance enters the system which could lead to errors in result. Hence, total mass of flask remains intact. It neither decreases nor increases. 
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