Internal energy ideal gas

But the pure ideal-gas internal energy and enthalpy are independent of pressure and volume, so these reduce to... 

In all these equations (4.2.2)-(4.2.9), the mixture and each pure ideal gas must be at the same temperature, pressure, and composition, except the ideal-gas values for U and H, which only must be at the same temperature and composition because the ideal-gas internal energy and enthalpy are independent of pressure. 

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