Question

what happens to the reported percent water if the salt thermally decomposes yielding a volatile product

Answer 1

water content should increase because say you have 10 % water content (10 unit per 100 unit of salt) after thermal decomposition the mass of salt will decrease say to 90 units than the water content will be 10/90. which is 11.11 %. I have assumed that the decomposition takes well below the boiling point of water (100 C) so the water content does not change