Quick question about perfect gas

[criticalPoint]

Two different gasses (Helium and Cripton) are mixed up. We can assume the compound behaves as a perfect gas only if the atoms of He and Kr have the same average value of:

• Mass
• Momentum
• Velocity
• Kinetic energy

Which one is correct? I can't figure it out.

 

[Jamison Lahman]

Which one do you think it is and why? Let's start there.

 

[criticalPoint]

Obviously it's not mass (average mass can't be equal in any circumnstances), because they are at the same temperature I suppose they have the same kinetic energy, but wouldn't that happen for every kind of mixture? I can't undestand what specific property of a perfect gas is important here, what makes this mixture perfect and not real.

 

[Jamison Lahman]

Obviously it's not mass (average mass can't be equal in any circumnstances)

I agree about mass.

because they are at the same temperature I suppose they have the same kinetic energy, but wouldn't that happen for every kind of mixture? I can't undestand what specific property of a perfect gas is important here, what makes this mixture perfect and not real.

Not necessarily. Maybe after a long period of time every mixture will be in thermal equilibrium, but it doesn't have to be the case. I agree thermal equilibrium is a property which would imply the same kinetic energy. What are the other properties? Can they help us here?