If a gas obeys the equation of state $\mathrm{P}(\mathrm{V}-\mathrm{nb})=\mathrm{nRT}$, the ratio $\frac{\left(\mathrm{C}_{\mathrm{P}}-\mathrm{C}_{\mathrm{V}}\right)}{\left(\mathrm{C}_{\mathrm{P}}-\mathrm{C}_{\mathrm{v}}\right)_{\text {ideal }}}$ is: