Two cylinders A and B of equal capacties  are connected to each other via a stopcock .A contains a gas at STP. B is completely evacuated.The entire system is thermally insulated.The stop cock is suddenly opened.Answer the following :

 

As the gas undergoes free expansion and walls of the container are made of insulating material , W = 0 and Q = 0. Therefore from Q=$△U +W , we can write △U = 0 .$ the internal energy remains constant so temperature remains constant. But as the volume of gas is increasing its pressure decreases.