Given that the gas constant R = 8.3145 J mol⁻¹K⁻¹ = 0.08205 L atm mol⁻¹K⁻¹. , show units in numerical work. An unknown substance has an enthalpy of vaporization of 15.3 kJ/ mol at 177 K, which is its normal boiling point. The liquid substance has a density of 0.792 g/mL. Determine the molar mass of the unknown substance.