The dependence of the detonation velocity on the initial density is available for several condensed high explosives. It gives a collection of Rayleigh lines on a pressure-volume plane starting from different initial volume points. It is shown that a collection of the Chapman-Jouguet (C-J) states on each Rayleigh line can be described by an envelope function of Rayleigh lines. Based on the analytical expression of the envelope function, formulae for other C-J parameters are derived. Calculated C-J state curves for several explosives on pressure-volume plane are found to fit very well with the experimental data for C-J pressure data. A detailed comparison of the calculated values of various other C-J parameters with the experimental data revealed that the discrepancy between the present approximation and the data depends on the choice of the parameter set to compare. Although the best match has been given for the p-v plane, but on other planes, the agreement is within a few to ten percent in case of PETN. A plot of the estimated C-J pressure vs. measured pressure for various explosives shows that the present model gives the C-J pressure at least within ten percent. Thermodynamic analysis based on the Jones-Stanyukovich-Manson relation has revealed that the present approximation is equivalent to the assumption that the Gruneisen parameter equals zero.