Partial molar quantities in an ideal gas mixture

We need to relate the chemical potential of a constituent of a gas mixture to its partial pressure. We cannot measure the absolute value of a chemical potential, but we can evaluate its value relative to the chemical potential in a particular reference state called the standard state. 

The standard state of substance i in a gas mixture is the same as the standard state of the pure gas described in Sec. 7.7 It is the hypothetical state in which pure gaseous i has the same temperature as the mixture, is at the standard pressure p°, and behaves as an ideal 

Thermodynamics and Chemistry, second e6V. on, neis on 3 20 by Howard DeVoe. Latest version www.chem.umd.edu/thermobook 

To derive an expression for /r, in an ideal gas mixture relative to we make an 

Our assumption, then, is that the partial pressure Pa of gas A in an ideal gas mixture in equilibrium with pure ideal gas A is equal to the pressure of the pure gas. 

---