New quasi-chemical models are developed to estimate semi-quantitatively the equilibrium constants of hydrate formation, which also explain the parabolic-shaped dependences of sound speed or compressibility on concentration of non-electrolytes in their aqueous solutions. It was shown that only two fitted parameters, the hydration number n(h) and the formation constant of the hydrate beta, are able to reproduce satisfactorily the experimental data. The results obtained are spectacular and consistent with former observation concerning hydration of non-electrolytes. (C) 2014 Elsevier B.V. All rights reserved.