Tutoring chemistry, definitions are important. The tutor mentions the term hygroscopic.
hygroscopic (adj):readily collecting water, including water vapor.
A sample of hygroscopic material can draw in water from the air, thereby gaining mass invisibly as it sits, uncovered, on the desk. Therefore, its number of moles may not be relatable to its mass, because of the unknown mass of water it may contain.
Source:Oracle Tutoring by Jack and Diane, Campbell River, BC.