where γ is the heat capacity ratio (γ ¼ CP/CV). All quantities in the above relation : Vm, γ and ð∂P=∂VmÞT can be calculated from a PVT relation(e.g.,a cubic EOS),in the case of γ, the isobaric molar heat capacity of the perfect gas is required. Fig. 4 shows calculated speed of sound for three hydrocarbons versus temperature and at atmospheric pressure. However, such equations of state show poor performance for the density and compressibility of liquid systems, especially for heavy hydrocarbons where the input parameters can not be estimated accurately (Shabani etal.,1998). A modified cubic equation of state in which the co-volume parameter b was related to molar refraction Rm instead of acentric factor has been proposed for heavy hydrocarbons (Riazi and Mansoori,1993 b). The modified equation of state was shown to provide a good description of the density of hydrocarbon liquids, both pure and mixed, including long-chain alkanes up to n-Tetracontane(C40). In this EOS, molar refraction defined below was used as the third dividing Rm of a compound to its value for methane: