`Na_(2)CO_(3).H_(2)O` on exposure to air loses nine water moleclues from its water of crystallization because initially vapour pressure of the compound is more than vapour pressure of the moisture present in the atmospheric air. It is an example of an efflorescent substance. After the release of nine molecules of water of crystallization its vapour pressure becomes equal to the pressure of the moisture present in atmosphere. Hence removal of 10th water molecule requires.