if the gas was not pure butane but 10 mole

Question

If the gas was not pure butane but 10 mole percent propane and 90 mole percent butane then determine how would the MW value be affected?

Using an ideal gas and its n moles are expanded adiabatically and reversibly from P₁, V₁, and T₁ to a final state V₂. It is given that the molar heat capacity at constant volume = 1.5, find an algebraic equation for P₂ in terms of the other variables (including 'n').