Four water molecules. $\mathrm{C}\mathrm{u}\mathrm{S}{\mathrm{O}}_4.5{\mathrm{H}}_2\mathrm{O}$ is a crystalline salt. Four ${\mathrm{H}}_2\mathrm{O}$ molecules attach to copper forming a square planar symmetry and two oxygen atoms from $\mathrm{S}{\mathrm{O}}_4^{2-}$ ion complete the distorted octahedron. The fifth water molecule is attached through hydrogen bonding between one of the co-ordinated water molecule and one of the sulphate ion.