A form for the Clausius-Clapeyron vapor-pressure equation is obtained in the Pitzer corresponding states scheme. This equation allows one to rewrite the well-known Trouton, Guldberg, van Laar and Guggenheim rules in terms of the acentric factor x. The original forms of these empirical rules are recovered for some particular values of x. The proposed rules are checked by analyzing National Institute of Standards and Technology (NIST) data on the liquid-vapor coexistence curve for 105 fluids. These rules have been also analyzed by using the well-known Ambrose-Walton (AW) vapor pressure equation. (C0 2013 Elsevier Ltd. All rights reserved.