Hydrates & Gram Formula Mass. Hydrates:. Hydrate = a substance in which water is part of the crystal structure. Feels dry to the touch Water is “trapped” Naming = name of the compound + “prefix” hydrate AlCl 3 •6H 2 O K 2 SO 3 •2H 2 O Ni(OH) 2 •H 2 O. Naming Hydrates:.