In much of the theoretical work on polymers in dilute solution the effects of solvent are treated in an implicit fashion: thus one studies an isolated chain interacting via an effective site-site potential. Although a formally exact mapping is possible between the chain-in-solvent system and a corresponding isolated effective-potential-chain, this mapping involves a many-site solvation potential which is not practical to compute. Thus, one generally resorts to a two-site potential approximation. In this article we test the validity of this two-site approximation for a short hard-sphere (HS) n-mer chain (n < 5) in a HS solvent. This test involves computing a set of "exact" site-site solvation potentials for an isolated chain. These potentials are shown to reproduce the conformation of a chain in explicit solvent. We make use of these exact short-chain results (combining them with the solvent potential of mean force) to construct approximate two-site solvation potentials for long chains. Monte Carlo simulations have been performed for HS chain-in-solvent systems and for the corresponding isolated effective-potential n-mer chain with n <= 50. These simulations show that our solvation potentials provide a quantitatively accurate description of the conformation of a HS chain in explicit (hard sphere) solvent. (c) 2007 Wiley Periodicals, Inc.