(a) When a stopcock is suddenly opened, the volume available to the gas at 1 atmosphere pressure will become two times. Therefore, pressure will decrease to one half, i.e., `0.5` atmosphere.
(b) There will be no change in the internal energy of the gas as no work is done on//by the gas.
(c) Also, there will be no change in temperture of the gas as gas does not work in expansion.
(d) No, because the process called free expansion is rapid and cannot be controlled. The intermediates states are non-equilibrium staes and do not satisfy the gas equation. In due course, the gas does return to an equilibrium state.