[noory]

A mixture of NO and O2 in a 1o-L container has a pressure of 8,20 atm at 227 degrees.The gases are reacted to give NO2.In the reaction O2 is in excess. The final pressure after completion of reaction is 6,15 atm at 227 degrees.What is the mole fraction of NO before reaction

[Yggdrasil]

Always start with a balanced chemical reaction.  Also, use the ideal gas law to figure out how many moles of gas are present before and after the reaction.