1.5 kg of Oxygen (N₂) and 0.15 kg of Helium (He₂) are separated by a partition in a vessel.  The Nitrogen is at 5 bar and 90^oC and the Helium at 1 bar and 20^oC.  The partition is removed and the gases are allowed to mix.

Find,

(1) The mass fraction, mole fraction and mean molecular weight of the mixture,

(2) The temperature and pressure after mixing,

(3) The entropy change in the mixing process.

The c_v values for Oxygen and Helium are 0.665 kJ/kgK and 3.116 kJ/kgK respectively.  (O₂ has a molecular weight of 32; He₂ has a molecular weight of 4).