Activity is defined as the ratio of this quantity's present value to its standard state value.  
If a gas obeys the van der Waals equation of state, then this quantity can be calculated at any temperature.  This quantity related to the chemical potential can be used to determine phase equilibrium for mixtures.  Activity is defined as the ratio of this quantity's present value to its standard state value.  For a gas, it represents the pressure a real gas would exert if it were ideal.  FTP, identify this term from physical chemistry that measures the tendency of a molecule to expand or escape its current phase.
If a gas obeys the van der Waals equation of state, then this quantity can be calculated at any temperature.  
fugacity