The formation energies of H bonds between ice surface OH groups with H-2, N-2, and CO were calculated from Iogansen's empirical "rule of intensities". They are in reasonable agreement with other experimental and theoretical estimations and close to those of surface OH groups of silica with the same gases. The latter comparison facilitates a quantitative estimation of the acidity of surface ice OH group relative to that of silica surface OH groups and to that of other H-bonding acids. The acidity of surface ice OH groups is about twice that of the OH group of a free water molecule, as a consequence of the cooperative effect.