Based on the improved Rosen-Morse oscillator for describing the internal vibration of a molecule, we establish an explicit representation of molar entropy for gaseous substances. Using the dissociation energy, equilibrium internuclear distance and harmonic vibrational frequency, we calculate the molar entropy values of the gaseous phosphorus dimer. Our calculated results show an excellent agreement compared to the experimental data in a wide variety of temperatures ranging from 298 up to 6000 K. (C) 2018 Elsevier Ltd. All rights reserved.