The BWRS approach is based on molar volume = 1/rho. As such, to apply the Starling entropy departure equation, you start with $C_v^{IG}\ln{(T2/T1)}$ and apply the Starling entropy departure corrections to this.

The BWRS approach is based on molar volume = 1/rho. As such, to apply the Starling entropy departure equation, you start with $C_v^{IG}\ln{(T2/T1)}$ and apply the Starling entropy departure corrections to this.

Alternately, you can start with the full ideal gas relationship $C_v^{IG}\ln{(T_2/T_1)}-R\ln{(\rho_2/\rho_1)}$ and then apply the Starling entropy departure equation, with the first term omitted because it represents the ideal gas contribution of molar volume.