If the gas is ideal, both the initial (, , ) and final (, , ) conditions follow the Ideal Gas Law, so that initially
and then, after the tap is opened,
Here  is the number of moles of gas and  is the molar ideal gas constant. Because the internal energy does not change and the internal energy of an ideal gas is solely a function of temperature, the temperature of the gas does not change; therefore . This implies that
Therefore if the volume doubles, the pressure halves.
The fact that the temperature does not change makes it easy to compute the change in entropy of the universe for this process.