Gibbs energy for ideal gas

According to Equation 5.44, from the Maxwell relations we obtain 

We further assume that we are dealing with an ideal gas and apply the ideal gas law  

If the pressure changes for constant temperature, AG changes as follows  

In other words, free energy is related to pressure by a logarithmic function. If there are several components, we have to use partial pressure. Furthermore, partial pressure is proportional to the concentration of the substance in the fluid phase. Equation 5.79 may therefore be used for the fluid phase if we replace the pressure with concentration in Equation 5.79. 

---