An insulated rigid tank that contains 1 kg of O2 at 25oC and 500 kPa is connected

Question:

An insulated rigid tank that contains 1 kg of O2 at 25oC and 500 kPa is connected to another insulated rigid tank that contains 1 kg of O2 at 25oC and 1000 kPa. The valve connecting the two tanks is opened, and the two gases are allowed to mix adiabatically. Determine
The entropy generated (sgen) during mixing per unit mass of the mixture.