- A: By pV = nRT, before the tap S is open, they have the same n (number of molecule), so (A) is correct.
- B: The average K.E. of the two same gas is only depend on its temperature, since two vessal have same temperature, so the average K.E. are same, so (B) is correct.
- C: The gas will flow from low pressure to the high pressure, so the flow is from X to Y, so (C) is correct.
- D: The pressure after the equilibrium can be find by pV = nRT, the new pressure should be (2n)RT/(3V) = (2/3)p, so D is incorrect.