In this work, we accurately calculate chemical potentials of organic compounds in dense liquid phases by using expanded ensemble Monte Carlo simulations. To facilitate the insertion of a solute molecule, Lennard-Jones size parameters and bond lengths are varied with coupling parameter. A new adaptive scheme is proposed to determine biasing weights during the simulation, which greatly enhances the efficiency and applicability of the expanded ensemble method. The chemical potentials of organic molecules of moderate sizes are obtained from a single run of simulation. The chemical potentials of several hydrocarbon molecules including n-alkanes, benzene and toluene in their pure liquid phases as well as in aqueous phases are calculated and compared with experimental values.