I haven't heard about a general law or formula. You cam estimate two margins. The density (or specific gravity - with this crude estimation it does not make a difference) it is less than: SG<(M(H2O)+M(Z))/VH2O. The second margin: SG>(M(H2O)+M(Z))/(V(H2O)+V(Z)). More accurate method is to use empirical formula, sometime you can find them for particular salts, or generate linear or quadratic approximation from handbook data.